Download Types of Stars http://space.about.com/od/stars/tp/What-Are

Types of Stars
http://space.about.com/od/stars/tp/What-Are-The-Different-Types-Of-Stars.htm
When we think of stars we usually imagine our Sun. But the simple fact is that the Universe
is made up of many different types of stars. In fact looking into the heavens most stars look
very different, and some aren't really stars at all. So here is a run down of the most common
types of stars in the galaxy.
1. Main Sequence Stars
“
”
A star is said to be born once nuclear fusion commences in its core. At this point it is,
regardless of mass, considered a main sequence star. This is where the majority of a star's
life is lived. Our Sun has been on the main sequence for about 5 billion years, and will
persist for another 5 billion years or so before it transitions to become a Red Giant Star.
2. Red Giant Stars
“
”
Once a star has used up all of its hydrogen fuel in its core it transitions off the main
sequence and becomes a red giant. Depending on the mass of the star it can oscillate
between various states before ultimately becoming either a white dwarf, neutron star or
black hole. One of our nearest neighbors (galactically speaking), Betelgeuse is currently in
its red giant phase and is expected to go supernova at any time.
3. White Dwarfs
“
”
When low-mass stars, like our Sun, reach the end of their lives they enter the red giant
phase. But the outward radiation pressure overwhelms the gravitational pressure and the
star expands farther and farther out into space. Eventually, the outer envelope of the star
begins to merge with interstellar space and all that is left behind is the remnant of the star's
core. This core is a smoldering ball of carbon and other various elements that glows as it
cools. While often referred to as a star, a white dwarf is not technically a star as it does not
undergo nuclear fusion. Rather it is a stellar remnant, like a black hole or neutron star.
Eventually it is this type of object that will be the sole remains of our Sun billions of years
from now.
4. Neutron Stars
“
”
A neutron star, like a white dwarf or black hole, is actually not a star but a stellar remnant.
When a massive star reaches the end of its life it undergoes a supernova explosion, leaving
behind its incredibly dense core. A soup-can full of neutron star material would have about
the same mass as our Moon. There only objects known to exist in the Universe that have
greater density are black holes.
5. Black Holes
“
”
Black holes are the result of very massive stars collapsing in on themselves due to the
massive gravity they create. When the star reaches the end of its main sequence life cycle,
the ensuing supernova drives the outer part of the star outward, leaving only the core
behind. The core will have become so dense that not even light can escape its grasp. These
objects are so exotic that the laws of physics break down.
6. Brown Dwarfs
“
”
Brown Dwarfs are not actually stars, but rather "failed" stars. They form in the same
manner as normal stars, however they never quite accumulate enough mass to ignite
nuclear fusion in their cores. Therefore they are noticeably smaller than main sequence
stars. In fact those that have been detected are more similar to the planet Jupiter in size,
though much more massive (and hence much denser).
7. Variable Stars
“
”
Most stars we see in the night sky maintain a constant brightness (the twinkling we
sometimes see is actually an atmospheric effect and not a variation of the star), but some
stars actually do vary. While some stars owe their variation to their rotation (like rotating
neutron stars, called pulsars) most variable stars change brightness because of their
continual expansion and contraction. The period of pulsation observed is directly
proportional to its intrinsic brightness. For this reason, variable stars are used to measure
distances since their period and apparent brightness (how bright they appear to us on
Earth) can be sued to calculate how far away they are from us.
In the table below, four types of stars are compared to our Sun.
8. Protostar
A protostar is what you have before a star forms. A protostar is a collection of gas that has
collapsed down from a giant molecular cloud. The protostar phase of stellar evolution lasts
about 100,000 years. Over time, gravity and pressure increase, forcing the protostar to
collapse down. All of the energy release by the protostar comes only from the heating
caused by the gravitational energy – nuclear fusion reactions haven’t started yet.
9. T Tauri Star
A T Tauri star is stage in a star’s formation and evolution right before it becomes a main
sequence star. This phase occurs at the end of the protostar phase, when the gravitational
pressure holding the star together is the source of all its energy. T Tauri stars don’t have
enough pressure and temperature at their cores to generate nuclear fusion, but they do
resemble main sequence stars; they’re about the same temperature but brighter because
they’re a larger. T Tauri stars can have large areas of sunspot coverage, and have intense Xray flares and extremely powerful stellar winds. Stars will remain in the T Tauri stage for
about 100 million years.
10. Red Dwarf Star
Red dwarf stars are the most common kind of stars in the Universe. These are main
sequence stars but they have such low mass that they’re much cooler than stars like our
Sun. They have another advantage. Red dwarf stars are able to keep the hydrogen fuel
mixing into their core, and so they can conserve their fuel for much longer than other stars.
Astronomers estimate that some red dwarf stars will burn for up to 10 trillion years. The
smallest red dwarfs are 0.075 times the mass of the Sun, and they can have a mass of up to
half of the Sun.
11. Supergiant Stars
The largest stars in the Universe are supergiant stars. These are monsters with dozens of
times the mass of the Sun. Unlike a relatively stable star like the Sun, supergiants are
consuming hydrogen fuel at an enormous rate and will consume all the fuel in their cores
within just a few million years. Supergiant stars live fast and die young, detonating as
supernovae; completely disintegrating themselves in the process.
.
12. Binary Stars - Many stars in the universe are part of a multiple star system. A binary
star is a system of two stars that are gravitationally bound to each other. They orbit around
a common point, called the center of mass. It is estimated that about half of all the stars in
our galaxy are part of a binary system. Visual binaries can be seen as two separate stars
through a telescope. Spectroscopic binaries appear as one star and can only be detected by
studying the Doppler shifts on the star's spectrum. Eclipsing binaries are binary systems
where one star blocks the light from another as it orbits its companion.
References:
Want more information on stars? Here’s Hubblesite’s News Releases about Stars,
http://hubblesite.org/newscenter/archive/releases/star/
and more information from NASA’s imagine the Universe
http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html
http://outreach.atnf.csiro.au/education/senior/astrophysics/stellarevolution_formation.h
tml