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WHAT’S UP??
An introduction to the creatures of
the night sky
Begin
What’s Up?
There are some far out things that even the
ancients knew about. These include the
sun, moon, planets, stars, and comets.
More recently, other objects
have come into view.
The Sun
The Sun is our star.
It Contains 99% of
the mass of the solar
system. Nuclear
reactions in the Sun’s
core release the light
and energy needed for life on Earth. The Sun is
a 4.6 billion year old star of average size.
Learn more
The Moon
orbits around the Earth
at a distance of
240,000 miles. It is
smaller than the Earth
with a diameter about
the same as the
distance from
Boston to Seattle. The
moon’s surface is covered by craters, darker regions
called maria, and lighter regions called terrae. Learn
more
Planets
Five of the solar system’s
nine planets are visible to
the naked eye: Mercury,
Venus, Mars, Jupiter, and
Saturn. Earth, of course,
is under our feet. Planets
look different from stars.
They’re brighter, they
don’t twinkle, and they
change location on the
stellar background. Each
planet is an exciting world
in itself. Learn more
Stars
Stars are huge balls of ionized gas (plasma). They
are 75% hydrogen and 25% helium. Population II
stars are younger and contain significant quantities
of other elements. Population I stars are older and
do not. Stars vary greatly in size, temperature and
brightness (luminosity). Most fit into one of four
categories: supergiants, red giants, main-sequence
stars, and white dwarfs. They shine because
they’ve been heated by nuclear fusion reactions in
their cores. Learn more
Comets
Comets are dirty snowballs that fly through the
solar system. As they near the sun, heat and
charged particles push material off their cores
forming beautiful tails. Comets provide hints of
what our solar system looked like at its birth.
Learn more
Things get a bit Fuzzy
• Galileo observed the sky with a telescope for the first
time in 1609. He saw lots and lots more stars. Some
stars looked fuzzy. He wondered why.
• In the 1700’s, a fuzzy patch of sky was given the name
nebula (plural nebulae). It comes from a Latin word for
cloudy.
• In 1758, comet-hunter Charles Messier observed a
comet that didn’t move. He called it M1. He observed
many other similar objects. His catalogue eventually
included 110 “Messier Objects.”
• In 1864, The General Cattalogue of Nebulae was
published with 5,097 entries. Most of these were
discovered by William Herschel and his son John.
These fuzzy patches turned
out to be:
• Clusters – stars grouped close together within our own
galaxy, learn more
• Galaxies – collections of billions of stars very far away,
learn more
• Nebulae – clouds of gas between stars, learn more
• Other strange stuff – things for you to research
Clusters – stars grouped
close together within our
own galaxy
There are two different types of clusters
Globular Clusters
Open Clusters
Globular Clusters
contain between 100,000 and 1,000,000
gravitationally bound older stars. About
200 of them are evenly distributed about
the halo of our galaxy. They are 7-120 pc
in diameter and can be round or slightly
elliptical. They’ve helped us understand
the rotation of the galaxy and the evolution
of stars.
M13
Open Clusters
are smaller groups of gravitationally bound
stars. These stars are generally younger
and metal-rich. Open clusters orbit around
the center of our galaxy. They provide
important clues about stellar evolution and
the behavior of the Milky Way.
M 44
Galaxies – collections of
billions of stars very far
away
There are three types of galaxies, each
classified by its appearance. Our galaxy,
the Milk Way, is a spiral galaxy.
Spiral Galaxies
Elliptical Galaxies
Irregular Galaxies
Spiral Galaxies
Andromeda Galaxy
M 83
M 51 (Whirlpool Galaxy)
Elliptical Galaxies
M87
Irregular Galaxies
Large Magellanic
Cloud -
- Small Magellanic Cloud
Nebulae – clouds of gas
between stars
The space between stars is not empty. It is filled with the
Interstellar Medium (ISM).
More dense clouds of gas can be found in:
Corona (Circumstellar Gas)
Emission Nebula (HII regions)
Reflection Nebula (diffuse nebula)
Planetary Nebula
Supernova Remnants (SNR)
Dark Nebulae (Bok Globules)
Giant Molecular Clouds (GMC)
Interstellar Medium (ISM)
On average, there is a 2-3 pc space between stars in the
Milky Way. About 9% of the known matter in our galaxy
exists in the ISM. The ISM can be observed either as it
reflects light from nearby stars or as it absorbs their light.
Hydrogen accounts for 90% of the matter in the ISM,
sometimes in molecular clouds as big as 1,000 solar
masses. These clouds can be cold and dense. They
account for ½ the total mass of the ISM. Most of the gas
in the ISM is located on the Milky Way’s 4 spiral arms.
This gas is mostly charged. It is pushed out of our solar
system by the solar wind although some neutral particles
make it through to Earth. Outside of its clouds, the ISM
is very dilute and very hot. It is populated by molecules
and grains of dust.
DUST
Dust is made of very small (.01 to .001 micrometer)
needlelike silicates of Al, Fe, Mg, and graphite. Water,
methane, or ammonia is then frozen on, forming an outer
layer. Although it accounts for only 1-2% off all the
known matter in the galaxy, interstellar dust has a
profound effect on the light we receive from the sky. It
absorbs up to half the emitted starlight and reemits it in
the infrared. It provides a site for the bonding of the
hydrogen molecule, the most common molecule in the
universe. It linearly (and sometimes circularly) polarizes
incoming light. It is used by scientists to measure the
magnetic field in space. Dust is very important to the
chemistry of space.
Corona
(Circumstellar Gas)
is very hot (1,000,000 K) sparse gas emitted from the
surface of stars. It is ionized by collisions. Some of
the metals in this gas have lost several electrons.
our sun’s corona
Emission Nebulae
(HII regions)
are heated with UV
radiation by hot,
young, bright stars.
This radiation is
energetic enough to
ionize the hydrogen
in these clouds. The
nebulae then glow
red. They are hot,
10,000K, and are
active sights for star
formation.
M42 (Orion Nebula)
Eagle Nebula
NGC 2264
Rosette Nebula
Reflection Nebulae
are also called diffuse nebula. They are
Illuminated by white stars, but these clouds of
gas generally glow blue because blue light is
more easily scattered.
NGC7023
NGC 1999
M78
Planetary Nebula
Not where planets are formed!!
Planetary Nebula form around small (1-5 solar
mass) stars between their red giant and white
dwarf stages. They form as the red giant loses
its mass to the stellar wind and are therefore
generally rich in chemicals. Our sun will
generate a planetary nebula in about 5 billion
years. They are very small, typically only about
one light year across, and spherically shaped.
Their outer regions have expanded and cooled,
leaving them redder than the center.
M 57 (Ring Nebula)
NGC2392
M27 (Dumbell Nebula)
Supernova
Remnants (SNR)
are hot regions only a couple light years
across. They were formed as a
supernova’s shock wave rippled through
space collecting and heating all the matter
in its way. They are hot, slowly cooling
regions.
M1 (Crab Nebula)
Veil Nebula
Cassiopeia A
Cygnus Loop
Dark Nebulae
(Bok Globules)
Horsehead Nebula
are dark spots on the
night sky. They are
gravitationally bound
clouds of dust. These
regions are usually
about 1 pc across and
about 200 solar
masses. They can
lead to star formation.
Barnard 86
Giant Molecular Clouds
(GMC)
With a mass of 100,000
solar masses and a
diameter of 100pc, they are
the biggest things in the
Galaxy. Like nebulae, they
are more commonly found
close to the galactic center.
Some have hot cores.
When one star is formed in
a Giant Molecular Cloud, it
can produce a shock wave
that leads to the formation
of other stars and other
shock waves.
Other Strange Stuff
In the 20th century, scientists started
looking at the sky in different parts of the
electromagnetic spectrum. They saw
even stranger objects like:
quasars, blazars, neutron stars, white
dwarfs, black holes, pulsars, masers,
variable stars, extra-solar planets, etc.