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Facts About the Milky Way
How Big is the Solar System?
by Fraser Cain (Comments Off)
by Nicholos Wethington
Here are ten interesting facts about your home – well, at
least your home galaxy – the Milky Way.
1. It's warped.
The Milky Way is a disk about 120,000 light years
across (see the Guide to Space article on the diameter of
the Milky Way for more), with a central bulge that has a
diameter of 12,000 light years. The disk is far from
perfectly flat though, as can be seen in the picture below.
What warped it? Two of the galaxy's neighbors – the
Large and Small Magellanic clouds – have been pulling
on the dark matter in the Milky Way like in a game of
galactic tug-of-war. The tugging sets up a sort of
oscillating frequency that pulls on the hydrogen gas (of
which the Milky Way has lots of). Here's a more indepth article on Universe Today about How the Milky
Way got its Warp.
How big is the Solar System? It all depends on your
definition. Some scientists think that the furthest
influence of the Solar System extends out to 125,000
astronomical units (1 AU is the average distance from
the Earth to the Sun), or 2 light years. Since the nearest
star is 4.22 light-years away, the Solar System size could
extend almost half way to the nearest star.
Astronomers think that the Sun's gravitational field
dominates the gravitational forces of the other stars in
the Solar System out to this distance. Objects in this
region will fall towards the Sun, and not the other stars.
But is there anything actually out there? It's so far away
that scientists have no way of detecting any objects.
Astronomers believe that the Oort Cloud – the source of
long period comets – extends out to a distance of 50,000
AU, and maybe even 100,000 AU. Once again, though,
the Oort Cloud has never been seen directly. We just
guess that it exists because comets with extremely long
orbits sometimes pass near the Sun and then head back
out again. The Oort cloud could have a trillion icy
objects inside it.
Another boundary that separates the Solar System from
the rest of the galaxy is the heliosphere. This is the
region where the solar wind collides with the interstellar
medium and slows down. This region extends out to
about 95 AU, or 3 times the orbit of Pluto. No spacecraft
have ever reached the heliopause, but NASA's Voyager
spacecraft are expected to cross the region within the
next decade.
2. It has a halo, but you can't directly see it.
The Milky Way has a halo of dark matter that makes up
over 90% of its mass. Yes, 90%. That means that all of
what we can see (with the naked eye or telescopes)
makes up less than 10% of the mass of the Milky Way.
Now, it doesn't have a halo like those old cartoon
characters that die, sprout wings and play a harp in the
clouds. The halo is actually invisible, though we know it
exists by running simulations of what the Milky Way
would look like and how fast stars inside the galaxy's
disk orbit the center. The heavier it is, the faster they
should be orbiting. If you assume that the galaxy is made
up only of matter that we can see, then you get a rotation
rate for the stars that is well below what it should be, so
the rest is made up of what is elusively called "dark
matter," or matter that only interacts gravitationally (so
far as we know) with "normal matter".
To see some images of the probable dark matter density
distribution in our galaxy, check out The Via Lactea
Project.
3. It has over 200 billion stars
As galaxies go, the Milky Way is a middleweight. The
largest galaxy known, IC 1101, has over 100 trillion
stars, and other large galaxies can have more than a
trillion stars. Smaller galaxies like the aforementioned
Large Magellanic Cloud, have about 10 billion stars. The
Milky Way has between 200-400 billion stars, but when
you look up into the night sky the most you can see from
any one point on the Earth is about 2,500. We aren't
stuck with this many stars forever, though, because the
Milky Way is constantly losing stars – through
supernovae – and producing stars, netting about seven
stars per year.
The Milky Way wasn't always as it is today, a beautiful
barred spiral. It became its current size and shape by
eating up other galaxies. It's still doing so today – the
Canis Major Dwarf Galaxy is the closest galaxy to the
Milky Way because its stars are currently being added to
the Milky Way's disk, and our galaxy has consumed
others in its long history, such as the Sagittarius Dwarf
Galaxy.
6. Every picture you've seen of the Milky Way from
above is either another galaxy or an artist's
interpretation.
We can't take a picture of the Milky Way from above
(yet) because we are inside the galactic disk, about
26,000 light years from the galactic center. This means
that any pretty pictures you see of a spiral galaxy with
elegant arms that is supposedly the Milky Way is either
a picture of another spiral galaxy, or the rendering of a
talented artist. Imaging the Milky Way from above is a
long, long way off; however, this doesn't mean that we
can't take breathtaking images of the Milky Way from
our vantage point!
7. There is a black hole at the center.
4. It's really dusty and gassy.
You may not think so by looking at it, but the Milky
Way is full of dust and gas. And when I say full of dust,
I mean that we can only see out about 6,000 light years
into the disk of our own galaxy in the visible spectrum,
and the galaxy is about 100,000 light years across! The
dust and gas makes up a whopping 10-15% of the
"normal matter" in the galaxy, with the remainder being
stars. The thickness of the dust deflects visible light, as
is explained here, but infrared light can pass through the
dust, which makes infrared telescopes like the Spitzer
Space Telescope extremely valuable tools in mapping
and studying the galaxy. Spitzer can peer through the
dust to give us extraordinarily clear views of what is
going on at the heart of the galaxy and in star-forming
regions.
5. It's made up of other galaxies.
Most galaxies have a supermassive black hole at the
center. Ours is no exception. The center of our galaxy is
called Sagittarius A* (pronounced "A-star"), and it
houses a black hole with a mass of 40,000 Suns that is
14 million miles across (about the size of Mercury's
orbit). But this is just the black hole itself. All of the
mass trying to get into the black hole – called the
accretion disk – forms a disk that has a mass of 4 million
Suns, and would fit inside the orbit of the Earth. Though
like other black holes, Sgr A* tries to consume anything
that happens to be nearby, star formation has been
detected near this black hole behemoth.
8. It's almost as old as the Universe itself.
The most current estimate for the age of the Universe is
about 13.7 billion years. Our Milky Way has been
around for about 13.6 billion of those years, give or take
800 million years. The oldest stars in our the Milky Way
are found in globular clusters, and the age of the galaxy
is determined by taking the age of these stars, and then
extrapolating the age of what preceded them. Though
some of the constituents of the Milky Way have been
around for a long time, the disk and bulge themselves
didn't form until about 10-12 billion years ago, and the
bulge may have formed earlier than the rest of the
galaxy.
9. It's part of the Virgo Supercluster, a grouping of
galaxies within 150 million light years.
As big as it is, the Milky Way is part of an even bigger
structure called a supercluster. Superclusters are
groupings of galaxies on very large scales (100s of
millions of light years). In between these superclusters
are large voids of space where any space traveler would
encounter very little in the way of galaxies or matter.
Our close neighbors include the Large and Small
Magellanic Clouds, and the Andromeda Galaxy (the
closest spiral galaxy to the Milky Way), and along with
about 30 other galaxies this group of galaxies makes up
what is called the Local Group. But as you get further on
out, on the scale of hundreds of millions of light years,
the Milky Way can be seen to be just a small part of a
large grouping of galaxies 150 million light years in
diameter called the Virgo Supercluster.
10. It's on the move
The Milky Way, along with everything else, is moving
through space, and puts to shame anything from
everyday life that one could compare its speed to. The
Earth moves around the Sun, the Sun around the Milky
Way, and the Milky Way is part of the Local Group,
which is moving relative to the Cosmic Microwave
Background radiation – the radiation left over from the
Big Bang, which is a convenient reference point to use
when determining the velocity of things in the Universe.
The Local Group is calculated to move relative to the
CMB at about 600 km/s (2,200,000 km/h), which is
pretty darn fast!
Astronomical Units
by Jerry Coffey
even arbitrary unit; however, the use of AU to refer to
the astronomical unit is generally accepted. The
astronomical constant whose value is one astronomical
unit is referred to as unit distance and given the symbol
A.
Originally, the astronomical unit was defined as the
length of the semi major axis of the Earth's elliptical
orbit around the Sun. In 1976, the International
Astronomical Union revised the definition of the
astronomical unit for greater precision. The new
definition is the length for which the Gaussian
gravitational constant takes the value 0.01720209895
when the units of measurement are the astronomical
units of length, mass and time. An equivalent definition
is the radius of an unperturbed circular Newtonian orbit
about the Sun of a particle having infinitesimal mass,
moving with a mean motion of 0.01720209895 radians
per day, or that length for which the heliocentric
gravitational constant is equal to 0.017202098952
AU3/d2. It is approximately equal to the mean Earth–
Sun distance.
Very precise measurements of the relative positions of
the inner planets can be made by radar and by telemetry
from space probes. As with all radar measurements,
these rely on measuring the time taken for light to be
reflected from an object. These measured positions are
then compared with those calculated by the laws of
celestial mechanics: the calculated positions are often
referred to as an ephemeris and are usually calculated in
astronomical units. The International Astronomical
Union (IAU) currently accepted best estimate of the
value of the astronomical unit in meters is
149597870700(3) m, based on a comparison of JPL and
IAA-RAS ephemerides.
According to the ancient astronomer Eusebius,
Eratosthenes found the distance to the sun to be either as
4,080,000 stadia, or as 804,000,000 stadia. Using the
Greek stadium of 185 to 190 meters, the former
translation comes to a far too low 755,000 km whereas
the second translation comes to 148.7 to 152.8 million
km accurate within 2%. In modern use astronomical
units are used to make the numbers involved in
astronomy easier to use. Take for instance the distance to
Mars: it is 227,939,000 km from the Sun or 1.523 AU.
Astronomical units are a measurement that is equal to
the mean distance between the Earth and the Sun. The
International Astronomical Union currently estimates
that distance to be 150 million km. It is abbreviated AU,
au, or ua. In general, capital letters are only used for the
symbols of units which are named after individual
scientists, while au or a.u. can also mean atomic unit or
In astronomy, an astronomical unit is defined as the
average distance from the Sun to the Earth, or about 150
million kilometers (93 million miles). You can
abbreviate astronomical unit as AU.
Since the distances in astronomy are so vast,
astronomers use this measurement to bring the size of
numbers down. For example, Earth is 1 AU from the
Sun, and Mars is 1.523 AU. That's much easier than
saying that Mars is 227,939,000 km away from the Sun.
Using astronomical units is even more helpful when the
distances get larger. Pluto is 39.48 AU from the Sun, and
the newly discovered Eris is 67.67 AU. The Oort Cloud,
the source of long-period comets in the Solar System, is
thought to be 50,000 AU from the Sun.
Here are some handy conversions for you:
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1 astronomical unit = 149,598,000 kilometers
1 astronomical unit = 92,955,887 miles
1 light year = 63,239 astronomical units
1 parsec = 206,264 astronomical units
We have written many articles about measuring distance
in space. Here's an article about the distance to
Andromeda, and here's an article about the distance from
Earth to Mars.
Parallax
by Tega Jessa
measure the exact distance of stars. So how did they find
out? They did so using geometry and you can thank the
ancient philosopher Pythagoras for coming up with the
concept of using geometry to determine large distances.
If you remember from high school and middle school
math, Pythagoras came up with a theorem that you could
use any two sides of a triangle or one of its sides and one
of its angles to determine the exact length of the other
sides. This is how Pythagoras calculated the
circumference of the Earth.
The same principal was used to find the distance of a
star. First off it had to be close enough to Earth to see an
actual parallax. The further an object is the less it seems
to move when you change your point of view. By using
double the radius of the Earth's orbit as one side of a
triangle and the angle of the chosen star's parallax, you
could find out the exact distance to the star. This is
because both unknown sides of the cosmic triangle
would be the same, since the true distance did not
change, only the perspective. The formula boils down to
distance to the star equals the radius of Earth's orbit
divided by the tangent of half the angle of a stars
parallax.
This method works will for stars under 100 light years
away. This is due to the reason stated before that the
parallax of an object diminishes as it is placed further
away. The other reason is that the angle of parallax for
stars is often very small.
Parsec
by Stephanie Bender
So what is parallax? The best explanation is that it is the
illusion of an object moving when you look at it from
two different points of view. A perfect example is to
hold up your thumb and place it in front of other objects
for a frame of reference. Now try to looking at your
thumb with only your left eye. Now do the same thing
with your right eye. You will notice that it looked like
your thumb moved in reference to the other objects
behind it. Yet the entire time you know that you did not
move your thumb.
This visual principal also works in astronomy. If you
look at certain stars they seem to have different positions
in the night sky in relation to other stars depending on
the time of year. This is a stellar parallax. For the longest
time astronomers did not have the tools to physically
A parsec is equivalent to 3.26 light years. And since a
light year is the distance light travels in 1 year 9.4
trillion km, 1 parsec equals 30.8 trillion km. So, where
then, does the term, "parsec", come from? Parsec is a
combination of 2 words, parallax (par) and arcsecond
(sec).
Parallax means something looks like it changed its
location because you changed yours. For example, if you
stand on your porch and look across the street, you will
see a house on your left and a house on your right. If you
go across the street and look at the same houses from
your neighbor's backyard, they will be on the opposite
sides. Did the houses move? Of course not. You changed
your location. Since you are in a different place, facing a
different direction, they appear to be in different places.
Likewise, two different people, in two different parts of
the world, might see the exact same event in the sky or
outer space; yet, it might appear entirely different due to
their locations.
Astronomers measure parallax by measuring how distant
stars shift back and forth as the Earth travels around the
Sun. Astronomers measure the position of the stars at
one time of the year, when the Earth is at a position in its
orbit around the Sun, and then they measure again 6
months later when the Earth is on the other side of its
orbit. Nearby stars will have shifted a tiny amount
compared to more distant stars, and sensitive instruments
can detect the change.
Now for the second part of "parsec": arcsecond. In this
instance, we're not referring to a measure of time. It's a
part of a measurement of angle. Imagine the horizon
around you broken up into 360 slices, or degrees. Each
slice is about twice the width of the full moon. An
arcminute is 1/60th of a degree, and an arcsecond is
1/60th of an arcminute. So astronomers measure the size
of objects, or the parallax movement of stars in degrees,
arcminutes and arcseconds.
So, to put those terms together, a parsec is the parallax
of one arcsecond. Just a warning, you're going to need to
dust off your trigonometry for this. If you create a
triangle, where one leg is the distance between the Earth
and the Sun (one astronomical unit), and the opposite
angle, measured by how far the star moves in the sky, is
one arcsecond, the star will be 1 parsec away, the other
leg of the triangle.
For example, the closest star in the sky, Proxima
Centauri, has a parallax measurement of 0.77233
arcseconds – that's how far it shifts in the sky from when
the Earth shifts its position by 1 astronomical unit. If you
put this into the calculation, you determine that Proxima
Centauri is 1.295 parsecs away, or 4.225 light years.
All articles from Astronomy Today
(www.astronomytoday.com)