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Gas Planets
by Abby Cessna on October 26, 2009
the four gas giants in our Solar System Credit:
NASA/JPL
Gas planets are a category of planets without any solid
metals or rock. Gas planets are also referred to as gas
giants, giant planets, and Jovian planets. Since gas
planets do not have a solid surface, you would not be
able to walk on them. Gas planets are said to have a rocky center; however, that term is
somewhat misleading. Scientists believe that due to the high temperatures and extreme pressures
in the center of gas planets their rocky centers are actually liquid metal or rock. Thus, the density
of the gas planets simply increases as you go deeper to the center.
There are four gas planets in our Solar System: Jupiter, Saturn, Uranus, and Neptune. The other
four planets are known as terrestrial planets. The gas giants in our Solar System are also called
the outer planets because they are the four planets in the Solar System furthest from the Sun.
Although the gas planets are lumped into one category, there are also subcategories among them.
Astronomers differentiate between the “classic” or “traditional” gas giants and the “ice” giants.
The traditional gas giants are like Jupiter and Saturn. Jupiter is considered the model for
traditional gas giants. In fact, gas giants are sometimes also called Jovian planets after Jupiter.
They are comprised of mostly hydrogen and helium. While ice giants do have some hydrogen
and helium, they are mostly made up of “ices” such as methane, ammonia, and water. Methane is
what gives Uranus and Neptune their blue color. (Uranus and Neptune are the two ice giants in
our Solar System.)
Despite their differences, the gas planets in our Solar System do have quite a few similarities.
One of these similarities is that they all are much larger – have a much bigger volume – than the
terrestrial planets. Additionally, all of Solar System’s gas planets have planetary ring systems
and many moons. Because the gas giants are composed of gas, they are less dense than the
terrestrial planets. Their large size is what makes them more massive than the inner (terrestrial)
planets.
Gas planets do not just exist in our Solar System. Astronomers have discovered many gas giants
orbiting other stars. Hot Jupiters are a type of gas planets that astronomers have found. Very
similar to Jupiter in composition, the Hot Jupiters are closer to their suns, so they have much
higher temperatures. So far, astronomers have found more Hot Jupiters than any other type of
extra-solar planet.
Terrestrial Planets
by Abby Cessna on January 11, 2010
There are four terrestrial planets in our Solar System: Mercury, Venus, Earth, and Mars. The
Moon is not structurally the same as a terrestrial planet, but it does have some similarities. The
terrestrial planets in our Solar System are also known as the inner planets because these planets
are the four closest to the Sun. Terrestrial planets are also called rocky planets or telluric planets.
They differ from gas giants, the outer planets, in a number of ways.
Terrestrial planets share a number of common features. They are all composed mostly of rock
and heavy metals. These planets have a core made of heavy metals that is mostly iron; the core is
surrounded by a mantle of silicate rock. Terrestrial planets are much smaller than gas giants. The
terrestrial planets also have varied terrain such as volcanoes, canyons, mountains, and craters.
Another common feature among the terrestrial planets is that they have few or no moons.
Mercury and Venus have none while Earth has one. Mars has two small moons. Also, the
terrestrial planets do not have planetary rings like the gas planets do. The atmosphere of planets
can vary from Venus’ thick carbon dioxide atmosphere to almost nothing on Mercury.
Mercury is the smallest terrestrial planet in the Solar System. Its atmosphere is very thin, which
is why the planet alternates between burning and freezing temperatures. Mercury is also a dense
planet and is composed of mostly iron and nickel. Venus is also a terrestrial planet and has a
thick toxic atmosphere, which traps the heat making it the hottest planet in the Solar System. It is
nearly the same size as Earth.
Earth is the densest of the planets because of its high percentage of iron, even for a terrestrial
planet. Its atmosphere is composed mostly of oxygen, nitrogen, and carbon dioxide with traces of
other substances. Mars is the fourth terrestrial planet in the Solar System. Its surface is red
because of the iron in rocks that has rusted. Although the planet has a thin atmosphere, scientists
believe that it used to be thicker. They believe the atmosphere’s thinning caused any liquid water
the planet had to evaporate.
Ceres is considered a terrestrial dwarf planet. It has a rocky inner core with an icy mantle with as
much as one quarter of the planet water ice. The dwarf planet may also have a thin atmosphere
and has varied terrain like the other terrestrial planets. There are also a number of extrasolar
terrestrial planets that scientists believe they have identified, although they are much fewer than
extrasolar gas giants. Because they are so far away and smaller than gas giants, scientists have
been able to find very few extrasolar terrestrial planets.
What is an Exoplanet
by Tega Jessa on September 29, 2010
Hubble Finds Hidden Exoplanet in Archival Data
What is an exoplanet? An exoplanet is basically any
planet that is found outside the solar system. For the
longest time astronomers thought that planets only occurred in our solar system and even when
planets were discovered orbiting other stars that there were not any that had the same mass or
orbit as Earths. However new instrumentation and observation methods are proving this to be a
false assumption.
First how have we been able to detect planets outside our solar system? Planets if we use the
practical example of our own solar system are normally several orders smaller than their stars.
Since they also don’t emit light they are basically invisible to traditional telescopes. The way we
have found exoplanets is by math and observing the movements of stars. A star with planets and
a star without them will move differently because the subtle influence of each planet’s gravity.
While not having a gravitational field as strong as that of a star collectively the planets of a star
system do exert a pull on their respective stars. It was using a similar method observing the
movements of the outer planets that we found the remaining nine planetary objects in our solar
system.
In the case of stars spectroscopy is the method that was used spectroscopy and radial velocity.
Since planets exert a collective pull on their stars the stars end up moving in their own smaller
orbit. This movement causes a shift in the frequency of the light they emit due to the Doppler
Effect. With the help o f spectroscopy and some math scientist can find out if the movement of a
star is influenced by planets.
Another method of detecting exoplanets is observing changes in their brightness. It makes sense
that when a planet passes between a distant star and an observer on earth the brightness will
lessen momentarily. NASA launched a telescope that can measure this change in brightness and
use it to calculate the orbit of the planet as well as its mass.
The last method of detection is old fashioned radio telescope images. By detecting the radiation
emitted by distant celestial bodies scientist have also been able to find planets. The growing
consensus is that there are many more exoplanets in the universe than previously expected there
may be a lot more because the limitations of observations from Earth. This means interesting
thing for whether there is life on Earth. It used to be thought that Earth only had the right set of
conditions for life to be possible. However, there are more stars out there with planets the odds
of another planet with the ability to bear life being out there becomes more possible than ever
before.
What Is A Nebula?
by John Carl Villanueva on March 30, 2010
If we translate the Latin word ‘nebula’, it would simply mean
‘cloud’. The nebula that we are about to talk about, however, is so
much more than just a cloud. So what is a nebula?
A nebula is an interstellar cloud in outer space that is made up of dust, hydrogen and helium gas,
and plasma. It is formed when portions of the interstellar medium collapse and clump together
due to the gravitational attraction of the particles that comprise them.
For those who are not aware of this yet, outer space is not really totally a vacuum (although it
may sometimes be approximated as such). Rather, it is made up of gas and dust known
collectively as the interstellar medium or ISM. So, it is this dispersed matter that eventually
collapses and forms a nebula.
Nebulae, like the Orion Nebula, are often favorite astronomical objects of scientists who want to
learn more about stellar or planetary formation. You see, parts of a nebula may clump together
some more. The gravitational forces between particles is directly proportional to the their
masses, remember?
Thus, the more masses clump together, the greater their gravitational attraction will be to other
bodies and particles in their vicinity. As the particles clump further to form larger and more
massive structures, they attract more dust and gas. The pressure inside then gets so high that
nuclear fusion ensues. This results in the emission of high-energy electromagnetic radiation,
which in turn ionizes the outer layers of gas.
Ionized gas is plasma, and so plasma and electromagnetic radiation are now added to the mix.
This now becomes the earliest stages of star formation, and is what some scientists are most
interested about.
Actually, nebulae are not just the starting points of stellar evolution. Ironically, they can also be
the end points. You can think of this as the nebula-star-nebula cycle. Stars that evolve into red
giants can lose their outer layers during pulsations in their outer layers, known as their
atmospheres. This released matter is typically 97% hydrogen and 3% helium, with a few other
trace materials.
It is this released matter that forms what is known as a planetary nebula. The planetary nebula is
just one of four major types of nebulae. The other three are H II regions, supernova remnant, and
dark nebula.
Some of the more prominent nebulae are the Crab, Eagle, Orion, Pelican, Ring, and Rosette
Nebula.
What is a Black Hole?
by Nicholos Wethington on August 24, 2009
Artists concept of a black hole.
Black holes are scary, and some of the most intriguing and mysterious objects in the Universe.
This is probably why they are the subject of so many science fiction stories, astronomy articles,
and research papers. But what is a black hole? Black holes are essentially objects in space that
are so extremely massive and dense that nothing can escape their gravitational pull, including
light (which is why they are called black holes). This leads to some interesting scenarios
regarding the physics in the immediate area surrounding a black hole.
The theory of general relativity postulates that anything with mass curves the fabric of
spacetime, and this curvature is what we know as gravity. The most popular (and highly
descriptive) example to illustrate how this works is that of a rubber sheet. Imaging that you have
a large sheet of stretchy rubber that is stretched out reasonably taught. This sheet is a 2dimensional representation of the fabric of space (which has three dimensions, but for simplicity
and ease of illustration we’ll use two). If you place a small marble on the sheet, it will make a
small indentation in the rubber. Imagine that the marble is a planet, and if you have one of those
cool glass striped marbles, it may even look much like Jupiter. Rolling a small pebble (which
creates its own, very small indentation) by the marble, you may notice the pebble veers toward
the indentation of the marble. Essentially, the marble is pulling the small pebble towards it
because of its gravity.
Now, if you place a bowling ball on the sheet, which is much more massive, it makes a much
larger indentation that would likely trap the pebble as it rolled by. The bowling ball would
represent very well something like a star in this example. A black hole is all the mass of a star,
but in a very tiny space, enough to indent the rubber sheet enough so that nothing you roll by the
indentation of the black hole within a certain distance – no matter how fast you make it go – can
escape. The area around a black hole from which nothing can escape is called the event horizon,
and how big this area is depends on the size of the black hole itself.
A stellar black hole forms when a star that has a core above about 3 solar masses gets near the
end of its life, and the fusion processes inside the star are no longer pushing out sufficiently
against the inward pull of gravity, causing the star to implode. Once the matter inside the star is
compressed to below a certain radius – named the Schwarzschild Radius after the mathematican
who formulated it – a black hole is formed.
Structure of a black hole. Image Credit: NCSA
Supermassive black holes are those that form at the center of galaxies, and
they may range in the billions of solar masses. For example, the
supermassive black hole at the center of the Milky Way has a mass of about
40,000 Suns, and the matter that surrounds it – called the ‘accretion disk’ –
is a whopping 4 billion Suns. How do supermassive black holes form?
Here’s an excellent, detailed article on supermassive black holes that goes
over some of the competing theories about their formation.
Black holes come in different sizes depending on their mass. For example, if the Sun were to
become a black hole (it won’t, though, because it’s core is far too small) the radius of the black
hole would be about 3km (1.86 miles). If, through some weird set of circumstances, the Earth
were compacted into a space smaller than its Scwarzschild Radius, the black hole would be
roughly the size of a peanut.
At the center of a black hole lies what is called a singularity, where the mass of the black hole is
compressed to a volume of zero, and the ability for general relativity to describe what is
happening here breaks down.
How do we know black holes exist, if they don’t emit any light? Evidence for stellar black holes
comes from observing their interactions in binary systems, and supermassive black holes can be
observed using X-ray telescopes, as well as through the gravity they exert on the stars in a
galaxy.
Dark Matter
by Jean Tate on April 6, 2010
“Dark matter”, in astronomy, usually means “cold, nonbaryonic dark matter”. This is a form of mass which reacts
with other matter via only gravity – and, possibly, the weak
force – and which comprises approximately 80% of all matter in the universe. There is also
“baryonic dark matter”, which is just ordinary matter, like dust, gas, rocks, and even stars that
does not emit radiation yet detected by our telescopes (or absorb it, from more distant sources).
And there is also “hot, non-baryonic dark matter”, which is just neutrinos.
The first hints of the existence of dark matter came from an analysis of the line-of-sight
velocities of galaxies in the Coma cluster, by Fritz Zwicky, in the early 1930s. Zwicky found that
the galaxies are moving much too fast for them to be held together in a cluster, by gravity, if the
only mass in the cluster is that in the galaxies themselves (it’s pretty obvious that the galaxies
form a bound system). Since Zwicky could find no evidence of mass in the Coma cluster, from
the light detected by the telescopes he used, other than in the galaxies, he postulated that there is
a lot of matter that is ‘dark’ – does not emit light.
Fast forward to the early 1970s, and the discovery of diffuse x-ray emission from the Perseus and
Coma clusters.
Zwicky was right, the Coma cluster contains a great deal of mass outside the galaxies, and that
matter does not emit light (it emits x-rays), because it is very hot. But this thin plasma is still not
enough, mass-wise, to explain why the galaxies are gravitationally bound to the cluster (and the
Coma cluster is nothing special; today we know of thousands of clusters just like it). Further, the
plasma is also gravitationally bound to the cluster, but does not have enough mass itself to keep
it there. Some more mass is needed, and that mass is dark matter.
Around the same time, Kent Ford and Vera Rubin made a similar discovery, concerning spiral
galaxies; namely that they must contain a lot more matter than could be inferred from the stars,
gas, and dust observed by various telescopes, in order for the galaxies to be rotating as fast as
they are. Dark matter had been discovered in galaxies.
Types Of Galaxies
by Jerry Coffey on February 8, 2011
Optical image of The Andromeda galaxy (M31) (credit Robert
Gendler)
There are three basic types of galaxies: spirals, ellipticals, and
irregulars. The are classified based on the role of the bulge(round distribution of stars at the
center) and the disk(the flat distribution that includes the spiral arms). Ellipticals are all bulge
and not disk, irregulars lack symmetry and do not have a clear bulge or disk. Spirals have a clear
bulge, disk, and at least two ‘arms’ which are areas of new star formation(usually hot OB stars).
This is not to say that there are not further divisions of galaxies, but that these are the three mos
encompassing categories.
Elliptical galaxies are somewhat shaped like an American football and can contain trillions of
stars. Many scientists think that they are the result of two galaxies colliding. Most elliptical
galaxies are composed of old, low mass stars with a sparse interstellar medium and very little
new star formation. Ellipticals are surrounded by large numbers of globular clusters. They are
thought to make up 10–15% of galaxies in the local universe are commonly found near the center
of galaxy clusters.
Irregular galaxies does not have a distinct regular shape. The shape of an irregular galaxy does
not fall into any of the regular classes of the Hubble sequence and is often chaotic in appearance,
lacking a bulge, disk, or arms. This type is thought to make up 25% of all galaxies. Most
irregulars were once spiral or elliptical galaxies that were deformed by distortions in
gravitational pull.
Spiral galaxies make up about 35% of the galaxies in the known universe. They consist of a flat,
rotating disk that contains stars, gas, and dust along with a bulge full of stars. The bulge and disk
are surrounded by a halo. They are named from the spiral appearing arms jutting from the center
into the disk. Nearly half of the spiral galaxies have an additional component: a bar-like
structure, extending from the central bulge where the spiral arms begin.
You can see hints of further classification within these types of galaxies. There are at least ten
further categories of galaxies to look into. Good luck, and enjoy your research.
Galaxy Shapes
by Tega Jessa on June 8, 2011
Image credit: NASA/JPL-Caltech/UCLA
Science revealed to us that universe as we know it,
is composed of billions of galaxies like our own Milky Way. When you consider how many stars
are just in our own galaxy you can get just a small idea how big our universe really is. Despite
this astronomers have made great strides in learning more about the galaxies and their different
characteristics. One aspect that was defined early was their shapes. Thanks to the work of
famous astronomer Edwin Hubble we know that just about any galaxy in the universe will have
one of 4 different shapes, spiral, elliptical, lenticular, and irregular.
Spiral galaxies are one of the most familiar galaxy shapes. In fact when most people think of a
galaxy, this type of galaxy shape is the first to come to mind. This is because the Milky Way is a
prime example of a spiral galaxy. A spiral galaxy looks like a pinwheel. It is basically the
nucleus with its different “arms” spiraling outwards. Spiral galaxies can be tight or loose to
varying degrees. One important fact about spiral galaxies is that young stars are formed in the
outer arms while older stars are found near the center.
The next two types of galaxies are elliptical and lenticular shaped galaxies. These types are the
kinds that are the most similar. First they have few or no dust lanes and are largely composed of
older mature stars. These types seldom have star forming areas. Of the four galaxy shapes this is
the most cohesive and organized.
The final galaxy shape is the irregular galaxy shape. Irregulars have an indeterminate shape.
These galaxies are often small and don’t have enough gravitational force to organize into a more
regular form. The Hubble telescope has taken images of famous irregular galaxies like the
Magellanic Clouds. Irregular galaxies can also be large galaxies that have undergone a major
gravitational disturbance.
As you now see the four basic galaxy shapes seem to cover just about every type of galaxy out
there. Like any classification of shape there are also subcategories. An interesting observation
recently made about the shape of galaxies is the role that their formation plays in determining
their shape. It is now thought that galaxies get their shape as they naturally develop, merge with
other galaxies or disrupt each other’s path. This is another great mystery as we don’t currently
have the technology to plot out the complete paths of galaxies in the universe.
Galaxy Collision
by Fraser Cain on May 8, 2009
Colliding Galaxies. Image credit: Hubble
I don’t want to scare you, but our own Milky Way is on a collision course with Andromeda.
Some time in the next few billion years, our galaxy is going to crash into Andromeda with
catastrophic consequences. Stars will be thrown out of the galaxy, others will be destroyed as
they crash into the merging supermassive black holes. The delicate spiral structure of both
galaxies will be destroyed as they become a single, giant elliptical galaxy.
How do astronomers know this galaxy collision is going to happen? They’ve measured the
direction and speed of both galaxies, and calculated that it’s going to happen. But more
importantly, when astronomers look out into the Universe, they see galaxy collisions happening
everywhere.
Galaxies are held together by mutual gravity, orbiting a common center of gravity; imagine bees
buzzing around a beehive. Sometimes the galaxies get close enough to tear at each other with
their gravity. This gravitational interaction distorts the disk of galaxies and pulls off long tidal
tails; streams of stars connecting galaxies together. And if the galaxies get even closer, they’ll
collide.
In a galaxy collision, large galaxies absorb smaller galaxies entirely, tearing them apart and
adding their stars to the galaxy. But when the galaxies are similar sizes, like our Milky Way and
Andromeda, the close encounter destroys the spiral structure entirely. The two groups of stars
eventually become a giant elliptical galaxy with no discernible spiral structure.
Another consequence of galaxy collisions is star formation. When the galaxies collide, it also
causes vast clouds of hydrogen gas to collapse. This collapsing gas creates pockets of star
formation. A galaxy collision ages a galaxy prematurely, causing much of its gas to convert into
star formation. After this period of rampant star formation, galaxies run out of fuel. The youngest
hottest stars detonate as supernovae, and all that’s left are the older, cooler red stars with much
longer lives.
This is why the giant elliptical galaxies, the results of galaxy collisions, have so many old red
stars, and very little active star formation.
Structure of the
Universe
by John Carl Villanueva on August 15, 2009
Galaxy cluster Abell 85, seen by Chandra, left, and
a model of the growth of cosmic structure when the Universe was 0.9 billion, 3.2 billion and 13.7
billion years old (now). Credit: Chandra
The large-scale structure of the Universe is made up of voids and filaments, that can be broken
down into superclusters, clusters, galaxy groups, and subsequently into galaxies. At a relatively
smaller scale, we know that galaxies are made up of stars and their constituents, our own Solar
System being one of them.
By understanding the hierarchical structure of things, we are able to gain a clearer visualization
of the roles each individual component plays and how they fit into the larger picture. For
example, if we go down to the world of the very small, we know that molecules can be chopped
down into atoms; atoms into protons, electrons, and neutrons; then the protons and neutrons into
quarks and so on.
But what about the very large? What is the large-scale structure of the universe? What exactly
are superclusters and filaments and voids? Let’s start by looking at galaxy groupings and move
on to even larger structures.
Although there are some galaxies that are found to stray away by their lonesome, most of them
are actually bundled into groups and clusters. Groups are smaller, usually made up of less than
50 galaxies and can have diameters up to 6 million light-years. In fact, the group in which our
Milky Way is a member of is made up of only a little over 40 galaxies.
Generally speaking, clusters are bunches of 50 to 1,000 galaxies that can have diameters of up to
32 light-years. One very peculiar property of clusters is that the velocities of their galaxies are
supposed to be too high for gravity alone to keep them bunched together … and yet they are.
The idea that dark matter exists starts at this scale of structure. Dark matter is believed to provide
the gravitational force that keeps them all bunched up.
A great number of groups, clusters and individual galaxies can come together to form the next
larger structure – superclusters. Superclusters are among the largest structures ever to be
discovered in the universe.
The largest single structure to be identified is the Sloan Great Wall, a vast sheet of galaxies that
span a length of 500 million light-years, a width of 200 million light-years and a thickness of
only 15 million light-years.
Due to the limitations of today’s measuring devices, there is a maximum level to which we can
zoom out. At that level, we see a universe made up of mainly two components. There are the
threadlike structures known as filaments that are made up of isolated galaxies, groups, clusters
and superclusters. And then there are vast empty bubbles of empty space called voids.
Cosmic Microwave
Background Radiation
by Jerry Coffey on September 30, 2010
WMAP image of the Cosmic Microwave Background Radiation
Cosmic microwave background radiation(CMBR) is a form of electromagnetic radiation that
fills the universe. It can be visualized with a radio telescope. With an optical telescope, space
is black, but with a radio telescope there is a faint background glow, almost exactly the same
in all directions, that is not associated with any star, galaxy, or other object. This glow is
strongest in the microwave part of the spectrum. The CMBR is radiation left over from an
early stage in the development of the universe.
When the universe cooled enough, stable atoms formed. These atoms could no longer absorb
thermal radiation. The photons that existed at that time have been propagating ever since, but
they have been growing fainter and less energetic as the universe expands. This is the source for
the CMBR. Precise measurements of cosmic background radiation are critical to cosmology,
since any proposed model of the universe must explain this radiation. The CMBR has a black
body spectrum at a temperature of 2.725 K, so it peaks in the microwave frequency of 160.2
GHz, and a 1.9 mm wavelength.
The cosmic microwave background radiation is one of the main proofs of the Big Bang Theory.
Inflationary cosmology predicts that 10-37after the Big Bang, the universe underwent intense
growth that smoothed out nearly all inhomogeneities. This was followed by symmetry
breaking(phase transition that set the fundamental forces and elementary particles). Shortly after
the Big Bang, the early universe was made up of a hot plasma consisting of photons, electrons,
and baryons. As the universe expanded, the plasma was cooled by adiabatic cooling to the point
that electrons and protons could combine to form hydrogen. This happened when the universe
was approximately 379,000 years old.
Hundreds of cosmic microwave background experiments have been conducted to measure and
characterize the signatures of the radiation. The best known is probably the Cosmic Background
Explorer(NASA mission) satellite which detected and quantified the large scale anisotropies at
the limit of its detection capabilities. A series of ground- and balloon-based experiments
quantified CMBR anisotropies on smaller angular scales over the decade following the NASA
mission. These measurements were able to rule out cosmic strings as the leading theory of
cosmic structure formation and suggested cosmic inflation was correct. In June 2001 NASA
launched a second CMBR space mission to make much more precise measurements of the great
scale anisotropies over the full sky. The first results from this mission were detailed
measurements of the angular power spectrum to below degree scales, tightly constraining various
cosmological parameters. The long and short is that scientists have found a way to prove the Big
Bang Theory.
Thinking About Time Before
the Big Bang
by Nancy Atkinson on June 13, 2008
What happened before the Big Bang? The conventional answer to that question is usually,
“There is no such thing as ‘before the Big Bang.’” That’s the event that started it all. But the
right answer, says physicist Sean Carroll, is, “We just don’t know.” Carroll, as well as many
other physicists and cosmologists have begun to consider the possibility of time before the Big
Bang, as well as alternative theories of how our universe came to be. Carroll discussed this type
of “speculative research” during a talk at the American Astronomical Society Meeting last week
in St. Louis, Missouri.
“This is an interesting time to be a cosmologist,” Carroll said. “We are both blessed and cursed.
It’s a golden age, but the problem is that the model we have of the universe makes no sense.”
First, there’s an inventory problem, where 95% of the universe is unaccounted for. Cosmologists
seemingly have solved that problem by concocting dark matter and dark energy. But because we
have “created” matter to fit the data doesn’t mean we understand the nature of the universe.
Another big surprise about our universe comes from actual data from the WMAP (Wilkinson
Microwave Anisotropy Probe) spacecraft which has been studying the Cosmic Microwave
Background (CMB) the “echo” of the Big Bang.
“The WMAP snapshot of how the early universe looked shows it to be hot, dense and smooth
[low entropy] over a wide region of space,” said Carroll. “We don’t understand why that is the
case. That’s an even bigger surprise than the inventory problem. Our universe just doesn’t look
natural.” Carroll said states of low-entropy are rare, plus of all the possible initial conditions that
could have evolved into a universe like ours, the overwhelming majority have much higher
entropy, not lower.
But the single most surprising phenomenon about the universe, said Carroll, is that things
change. And it all happens in a consistent direction from past to future, throughout the universe.
“It’s called the arrow of time,” said Carroll. This arrow of time comes from the second law of
thermodynamics, which invokes entropy. The law states that invariably, closed systems move
from order to disorder over time. This law is fundamental to physics and astronomy.
One of the big questions about the initial conditions of the universe is why did entropy start out
so low? “And low entropy near the Big Bang is responsible for everything about the arrow of
time” said Carroll. “Life and death, memory, the flow of time.” Events happen in order and can’t
be reversed.
“Every time you break an egg or spill a glass of water you’re doing observational cosmology,”
Carroll said.
Therefore, in order to answer our questions about the universe and the arrow of time, we might
need to consider what happened before the Big Bang.
Carroll insisted these are important issues to think about. “This is not just recreational theology,”
he said. “We want a story of the universe that makes sense. When we have things that seem
surprising, we look for an underlying mechanism that makes what was a puzzle understandable.
The low entropy universe is clue to something and we should work to find it.”
Right now we don’t have a good model of the universe, and current theories don’t answer the
questions. Classical general relativity predicts the universe began with a singularity, but it can’t
prove anything until after the Big Bang.
Inflation theory, which proposes a period of extremely rapid (exponential) expansion of the
universe during its first few moments, is no help, Carroll said. “It just makes the entropy problem
worse. Inflation requires a theory of initial conditions.”
There are other models out there, too, but Carroll
proposed, and seemed to favor the idea of multiuniverses that keep creating “baby” universes. “Our
observable universe might not be the whole story,”
he said. “If we are part of a bigger multiverse, there
is no maximal-entropy equilibrium state and entropy
is produced via creation of universes like our own.”
Carroll also discussed new research he and a team of physicists have done, looking at, again,
results from WMAP. Carroll and his team say the data shows the universe is “lopsided.”
Measurements from WMAP show that the fluctuations in the microwave background are about
10% stronger on one side of the sky than on the other.
An explanation for this “heavy-on-one-side universe” would be if these fluctuations represented
a structure left over from the universe that produced our universe.
Carroll said all of this would be helped by a better understanding of quantum gravity. “Quantum
fluctuations can produce new universes. If thermal fluctuation in a quiet space can lead to baby
universes, they would have their own entropy and could go on creating universes.”
Granted, — and Carroll stressed this point — any research on these topics is generally
considered speculation at this time. “None of this is firmly established stuff,” he said. “I would
bet even money that this is wrong. But hopefully I’ll be able to come back in 10 years and tell
you that we’ve figured it all out.”
How Does a Star
Form?
by Fraser Cain on January 26, 2009
A star is formed out of cloud of cool, dense
molecular gas. In order for it to become a
potential star, the cloud needs to collapse and increase in density.
There are two common ways this can happen: it can either collide with another dense molecular
cloud or it can be near enough to encounter the pressure caused by a giant supernova. Several
stars can be born at once with the collision of two galaxies. In both cases, heat is needed to fuel
this reaction, which comes from the mutual gravity pulling all the material inward.
What happens next is dependent upon the size of the newborn star; called a protostar. Small
protostars will never have enough energy to become anything but a brown dwarf (think of a
really massive Jupiter). A brown dwarf is sub-stellar object that cannot maintain high enough
temperatures to perpetuate hydrogen fusion to helium. Some brown dwarfs can technically be
called stars depending upon their chemical composition, but the end result is the same; it will
cool slowly over billions of years to become the background temperature of the universe.
Medium to large protostars can take one of two paths depending upon their size: if they are
smaller than the sun, they undergo a proton-proton chain reaction to convert hydrogen to helium.
If they are larger than the sun, they undergo a carbon-nitrogen-oxygen cycle to convert hydrogen
to helium. The difference is the amount of heat involved. The CNO cycle happens at a much,
much higher temperature than the p-p chain cycle.
Whatever the route – a new star has formed.
The life cycle of a star is dependent upon how quickly it consumes hydrogen. For example,
small, red dwarf stars can last hundreds of billions of years, while large supergiants can consume
most of their hydrogen with a comparably short few million years. Once the star has consumed
most of its hydrogen, it has reached its mature state. This is how a star forms.
Size of Stars
by Fraser Cain on February 12, 2009
As you probably can guess, our Sun is an average
star. Stars can be bigger than the Sun, and stars can
be smaller. Let’s take a look at the size of stars.
The smallest stars out there are the tiny red dwarfs. These are stars with no more than 50% the
mass of the Sun, and they can have as little as 7.5% the mass of the Sun. This is the minimum
mass you need for a star to be able to support nuclear fusion in its core. Below this mass and you
get the failed star brown dwarfs. One fairly well known example of a red dwarf star is Proxima
Centauri; the closest star to Earth. This star has about 12% the mass of the Sun, and about 14%
the size of the Sun – about 200,000 km across, which is only a little larger than Jupiter.
Our own Sun is an example of an average star. It has a diameter of 1.4 million kilometers…
today. But when our Sun nears the end of its life, it will bloat up as a red giant, and grow to 300
times its original size. This will consume the orbits of the inner planets: Mercury, Venus, and
yes, even Earth.
An example of a larger star than our Sun is the blue supergiant Rigel in the constellation Orion.
This is a star with 17 times the mass of the Sun, which puts out 66,000 times as much energy.
Rigel is estimated to be 62 times as big as the Sun.
Bigger? No problem. Let’s take a look at the red supergiant Betelgeuse, also in the constellation
Orion. Betelgeuse has 20 times the mass of the Sun, and it’s nearing the end of its life;
astronomers think Betelgeuse might explode as a supernova within the next 1,000 years.
Betelgeuse has bloated out to more than 1,000 times the size of the Sun. This would consume the
orbit of Mars and almost reach Jupiter.
But the biggest star in the Universe is thought to be the monster VY Canis Majoris. This red
hypergiant star is thought to be 1,800 times the size of the Sun. This star would almost touch the
orbit of Saturn if it were in our Solar System.
The Death of a Star
Photo courtesy of NASA/Space Telescope Science Institute
Hubble Space Telescope photograph of the Rotten Egg
planetary nebula
Several billion years after its life starts, a star will die.
How the star dies, however, depends on what type of star it is.
Stars Like the Sun
When the core runs out of hydrogen fuel, it will contract under the weight of gravity. However,
some hydrogen fusion will occur in the upper layers. As the core contracts, it heats up. This heats
the upper layers, causing them to expand. As the outer layers expand, the radius of the star will
increase and it will become a red giant. The radius of the red giant sun will be just beyond the
Earth's orbit. At some point after this, the core will become hot enough to cause the helium to
fuse into carbon. When the helium fuel runs out, the core will expand and cool. The upper layers
will expand and eject material that will collect around the dying star to form a planetary nebula.
Finally, the core will cool into a white dwarf and then eventually into a black dwarf. This entire
process will take a few billion years.
Photo courtesy of NASA/Space Telescope Science Institute
Hubble Space Telescope photograph of the rings around Supernova 1987A
Stars More Massive Than the Sun
When the core runs out of hydrogen, these stars fuse helium into
carbon just like the Sun. However, after the helium is gone, their
mass is enough to fuse carbon into heavier elements such as oxygen,
neon, silicon, magnesium, sulfur and iron. Once the core has turned
to iron, it can burn no longer. The star collapses by its own gravity
and the iron core heats up. The core becomes so tightly packed that protons and electrons merge
to form neutrons. In less than a second, the iron core, which is about the size of the Earth, shrinks
to a neutron core with a radius of about 6 miles (10 kilometers). The outer layers of the star fall
inward on the neutron core, thereby crushing it further. The core heats to billions of degrees and
explodes (supernova), thereby releasing large amounts of energy and material into space. The
shock wave from the supernova can initiate star formation in other interstellar clouds. The
remains of the core can form a neutron star or a black hole depending upon the mass of the
original star.