Stars in galaxies
What is a Star?
Definition
A star is a huge ball of burning plasma that is held
together by gravity.
Stars are massive shining spheres of hot gas, the closest of
which is our Sun. Stars are primarily made of hydrogen, smaller amounts of
helium, and trace amounts of other elements.
The key to a star's existence is a phenomenon known as
hydrostatic equilibrium. The inward gravitational pressure created by the
star's mass is balanced by the outward radiation
pressure created by the nuclear fusion taking place in the core.
Even the most abundant of the other elements present in
stars are only present in very small quantities. Those stars which you see with
your naked eye in the night sky all belong to the Milky Way Galaxy, the huge
system of stars that contains our solar system.
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and
consists of hot plasma interwoven
with magnetic fields. It has a diameter of about 1,392,000 km, about 109 times
that of Earth, and its mass (about
2×1030 kilograms, 330,000 times that of Earth) accounts
for about 99.86% of the total mass of the Solar System. Chemically, about three
quarters of the Sun's mass consists of hydrogen,
while the rest is mostly helium. The remainder (1.69%, which nonetheless equals
5,628 times the mass of Earth) consists of heavier elements, including oxygen, carbon, neonand iron, among others.
Orbital characteristics
of the Sun
|
|
Mean distance
from Milky Waycore |
~2.5×1017 km
26,000 light-years |
Galactic period
|
(2.25–2.50)×108 a
|
~220 km/s (orbit around the center of the
Galaxy)
~20 km/s (relative to average velocity of other stars in stellar neighborhood) ~370 km/s[4] (relative to the cosmic microwave background) |
|
The Lifetime of a Star
The mechanism driving the outward radiation pressure in a
star's core is the nuclear fusion process where hydrogen is fused into helium
via the proton-proton chain. This reaction is exothermic, that is it produces
more energy than it takes to initiate the reaction.
Fusion is a natural process, but it is a difficult one to
achieve. It takes a tremendous amount of energy in order to initiate enough
fusion reactions to actually balance the force of gravity in a star.
Specifically, a star's core needs to reach temperatures in
excess of about 10 million kelvin to energize the hydrogen enough to fuse. Our
Sun, for instance has a core temperature around 15 million kelvin.
Therefore a star isn't said to have actually formed until
the core temperature reaches this level and fusion begins. Prior to this the
object is said to be a protostar.
Stellar Death
A star will continue existing on this primary part of its
life, known as the main sequence,
until it has used virtually all of the hydrogen fuel in its core. At this point
the core will contract because the outward radiation pressure is no longer
sufficient to balance the gravitational force.
This process, though, causes the core temperature to rise
allowing helium to fuse into carbon. At this point the star has expanded and
become a red giant.
The next phase in the star's evolution is completely
dependent on the mass of the star. If it is a low mass star, like our Sun, it
will eventually blow off its outer layers, creating a planetary
nebula with a white
dwarf in the middle.
High mass stars, however, will explode in a supernova. The
core of the original star is left behind as either a neutron star or a black
hole.
The Top 10 Brightest stars that we can see
Here are the Top 10 Brightest stars as seen from Earth.
1. Sirius
Sirius, also known as the Dog Star, is the brightest star in
the sky. Its name comes from the Greek word for scorching. At 8.7 light years,
it is also one of the closest stars to our own sun. In 1844, F.W. Bessel used
analysis of its motion to determine that it had a companion. This companion was
confirmed by observations in 1862 and is now known to be a white dwarf. Sirius
B, the companion, has received considerable attention itself, since it is the
first white dwarf with a spectrum to show a gravitational red shift as
predicted by the general theory of relativity. Sirius is located int the
constellation Canis Major.
Sirius Information
+Other Names: Dog Star, Canicula, Aschere, Alpha Canis Majoris, HR 2491, HD 48915.
+Data:
-Right Ascension: 06 45 08.9
-Declination: -16 42 58
-Apparent Magnitude: -1.46
-Absolute Magnitude: 0.00
-Distance: 8.6 LY
-Spectral Type: A1Vm
+Constellation: Canis
Major
2. Canopus
+Other Names: Suhel, Suhail, Alpha Carinae, HR 2326, HD 45348
+Data:
-Right Ascension: 06 23 57.1
-Declination: -52 41 45
-Apparent Magnitude: -0.72
-Absolute Magnitude: -2.5
-Distance: 74 LY
-Spectral Type: F0II
+Constellation: Carina
3. Rigil Kentaurus
Rigel Kentaurus, also known as Alpha Centauri, is the third
brightest star in the sky. Its name literally means foot of the centaur. It is
actually a triple star system made up of Alpha Centauri A, Alpha Centauri B,
and Alpha Centauri C (also known as Proxima Centauri because it is the closet
star to earth at 4.3 light years). Rigel Kentaurus is located in the
constellation Centaurus.
Rigil Kentaurus Information
+Other Names: Rigil Kent,Toliman, Alpha 1 Centauri, HR 5459, HD 128620.
+Data:
-Right Ascension: 14 39 35.9
-Declination: -60 50 07
-Apparent Magnitude: -0.27
-Absolute Magnitude: 4.4
-Distance: 4.3 LY
-Spectral Type: G2V
+Constellation: Centaurus
4. Arcturus
Arcturus is the brightest star in the constellation Bootes,
which is one of the oldest constellations in the night sky. It is the
4th-brightest star in the entire sky. Arcturus means bear guard, as it
overlooks the constellation Ursa Major. It is an orange giant with a diameter
about 10 times that of the sun and a luminosity about 100 times that of the
sun. At about 34 light-years, Arcturus is one of the nearest giant stars.
Arcturus is located in the constellation Bootes.
Arcturus Information
+Other Names: Haris-el-sema, Alpha Bootis, HR 5340, HD 124897
+Data:
-Right Ascension: 14 15 39.7
-Declination: +19 10 57
-Apparent Magnitude: -0.04
-Absolute Magnitude: 0.2
-Distance: 34 LY
-Spectral Type: 5IIIFe-0.5
+Constellation: Bootes
5. Vega
Vega is the fifth brightest star in the sky. Its name comes
from the Arabic for the swooping eagle. Vega is about 25 light-years from
Earth. It is three times the size of the sun and 50 times as luminous. Vega is
located in the constellation Lyra.
Vega Information:
+Other Names: Wega, Fidis, Harp Star, Alpha Lyrae, HR 7001, HD 172167
+Data:
-Right Ascension: 18 36 56.3
-Declination: +38 47 01
-Apparent Magnitude: 0.03
-Absolute Magnitude: 0.6
-Distance: 25 LY
-Spectral Type: A0Va
+Constellation: Lyra
6. Capella
Capella Information
+Other Names: Alhajoth, Alpha Aurigae, HR 1708, HD 34029
+Data:
-Right Ascension: 05 16 41.4
-Declination: +45 59 53
-Apparent Magnitude: 0.08
-Absolute Magnitude: 0.4
-Distance: 41 LY
-Spectral Type: G5IIIe+G0III
+Constellation: Auriga
7. Rigel
The seventh brightest star in the sky, Rigel's name is from
the Arabic for foot, indicating its place in the constellation Orion. It is a
blue supergiant and part of a 4 star system, consisting of 2 binary stars.
Rigel is approximately 1400 light-years from earth. Rigel is in the
constellation Orion.
Rigel Information
+Other Names: Algebar, Elgebar, Beta Orionis, HR 1713, HD 34085
+Data:
-Right Ascension: 05 14 32.3
-Declination: -08 12 06
-Apparent Magnitude:0.12
-Absolute Magnitude: -8.1
-Distance: 1400 LY
-Spectral Type: B8Ia
+Constellation: Orion
8. Procyon
Procyon is the eighth brightest star night sky. It is a
yellow-white star and at 11.4 light years, one of the closer stars to Earth.
Its name is from the Greek meaning before the dog, i.e., before the Dog Star,
Sirius, the brightest star in the sky. It is a visual binary star. Procyon is
in the constellation Canis
Minor.
Procyon Information
+Other Names: Elgomaisa, Algomeysa, Antecanis, Alpha Canis Minoris, HR
2943, HD 61421.
+Data:
-Right Ascension: 07 39 18.1
-Declination: +05 13 30
-Apparent Magnitude: 0.38
-Absolute Magnitude: 2.6
-Distance: 11.4 LY
-Spectral Type: F5IV-V
+Constellation: Canis
Minor
9. Achernar
The ninth brightest star night sky is Achernar. It is a
bluish-white white supergiant star that is about 69 light years from Earth. Its
name is from the Arabic meaning end of the river, in this case, the river being
Eridanus. Achernar is in the constellation Eridanus.
Achernar Information
+Other Names: Alpha Eridani, HR 472, HD 10144
+Data:
-Right Ascension: 01 37 42.9
-Declination: -57 14 12
-Apparent Magnitude: 0.46
-Absolute Magnitude: -1.3
-Distance: 69 LY
-Spectral Type: B3Vpe
+Constellation: Eridanus
10. Betelgeuse
Betelgeuse is the tenth brightest star in the sky. It is a
red supergiant about 13,000 times brighter than our sun and over 1000 times
larger. If you placed Betelgeuse in the place of our sun, it would extend past
the orbit of Jupiter. Its name is from the Arabic armpit, and is near the right
shoulder of Orion. It is nearing the end of its life and will soon become a
supernova. Betelgeuse is in the constellation Orion.
Betelgeuse Information
+Other Names: Betelguex, Betelgeuze, Beteiguex, Al Mankib, Alpha Orionis,
HR 2061, HD 39801
+Data:
-Right Ascension: 05 55 10.3
-Declination: +07 24 25
-Apparent Magnitude: 0.50(var.)
-Absolute Magnitude: -7.2
-Distance: ~1400 LY
-Spectral Type: M1-2Ia-Iab
+Constellation: Orion
References
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