What is Stars: Definition and 890 Discussions

A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but due to their immense distance from Earth they appear as fixed points of light in the sky. The most prominent stars are grouped into constellations and asterisms, and many of the brightest stars have proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable universe contains an estimated 1022 to 1024 stars, but most are invisible to the naked eye from Earth, including all individual stars outside our galaxy, the Milky Way.
A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. The total mass of a star is the main factor that determines its evolution and eventual fate. For most of its active life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space. At the end of a star's lifetime, its core becomes a stellar remnant: a white dwarf, a neutron star, or, if it is sufficiently massive, a black hole.
Almost all naturally occurring elements heavier than lithium are created by stellar nucleosynthesis in stars or their remnants. Chemically enriched material is returned to the interstellar medium by stellar mass loss or supernova explosions and then recycled into new stars. Astronomers can determine stellar properties including mass, age, metallicity (chemical composition), variability, distance, and motion through space by carrying out observations of a star's apparent brightness, spectrum, and changes in its position on the sky over time.
Stars can form orbital systems with other astronomical objects, as in the case of planetary systems and star systems with two or more stars. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.

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  1. B

    Stars move as the night progresses

    You know how the stars move as the night progresses. Well is it the declination that is varying with time? I don't think that it's the right ascension because with that you measure the angle from the vernal equinox to the point of intersection of the star's meridian with the celestial equator.So...
  2. turbo

    Mature stars in ancient galaxies

    http://arxiv.org/PS_cache/astro-ph/pdf/0502/0502385.pdf No kidding. Would anybody here like to hazard a guess at where this "reddening dust" lies on the path between the z~6 galaxies and our vantage point? If the "reddening dust" is very ancient and distant, I would be interested in hearing...
  3. Artermis

    Question: Mnemonic for classes of stars

    It's used to remember the order from brightest (hottest) stars to the dimmest (coldest stars). I remember that it starts with Oh,... then in the middle it maybe be: " Be It A Fine..." Boy/Girl, Kiss Me! So the order is like O ... B G K M It's driving me nuts; please help me!
  4. S

    Why Do Stars Flicker? Not Planets

    Stars are known t0 flicker. a) estimate the no. of photons entering the eye of an observer when he looks at the star of first apparent visual magnitude. Such a star produces flux on the surface of the Earth of 10^-6 lumens/sq.meter. One lumen is 0.0016 watts. Star Aldebaran is an example...
  5. G

    How Old Are Third Generation Stars

    In the Milky Way?
  6. wolram

    Pop 111 stars intermediat BHs

    http://arxiv.org/abs/astro-ph/0507593 Title: Core-Collapse Very Massive Stars: Evolution, Explosion, and Nucleosynthesis of Population III 500 -- 1000 $M_{\odot}$ Stars Authors: T. Ohkubo, H. Umeda, K. Maeda, K. Nomoto, S. Tsuruta, M. J. Rees Comments: 46 pages, 45 figure files We...
  7. B

    What Makes Neutron Stars So Unique?

    What are they? :confused:
  8. vincentm

    Exploring Red Supergiants: Size Limits and Birth Possibilities

    How big can these stars get, Also can a star be born at a Supergiant level? I'm having a hard time understanding a particular object. The Helix Nebula is said to be the result of a supernova explosion, a closeup reveals tiny knots of gas, now these knots are said to be at least the size of...
  9. T

    Why Variable Stars Change Magnitudes Periodically

    why variable stars have various magnitudes? and y they are changing periodically?
  10. G

    Question about exploding neutron stars

    I have a question about the time scale for a certain type occurance causing a neturon star to explode, and a related question about the conditions of this occurrance. If you have a binary star system with one of the stars being a neutron star, I read that if the other star sucks off enough...
  11. S

    Quasars,white dwarf and neutron stars

    quasars are highly active but extremely distant galaxies, right?what kind of galaxies are they and do they represent an earlier stage of the lives of the conventional galaxies?i've a vague idea that quasars a galaxies in which the supermassive black holes at the centres are actively gobbling up...
  12. Loren Booda

    Maximum count for mutually bound stars

    Especially in the early universe, what do you think would be the maximum number of stars bound in a system under mutual attraction?
  13. S

    Neutron Star Evolution and Possible Blackhole Formation in Proton Star Collapse

    Are Neutron Stars the major factors of Proton Stars? :http://arxiv.org/abs/astro-ph/0506092 Why do Neutron Stars evolve from Proton Star collapse, and what is the next evolving stage?..if any? :http://arxiv.org/abs/astro-ph/0506100 Can a further collapse occur that does 'not' produce...
  14. wolram

    Hyper Metal-Poor Stars: The First Generation in the Universe?

    http://arxiv.org/PS_cache/astro-ph/pdf/0505/0505524.pdf Title: The first chemical enrichment in the universe and the formation of hyper metal-poor stars Authors: Nobuyuki Iwamoto, Hideyuki Umeda, Nozomu Tominaga, Ken'ichi Nomoto, Keiichi Maeda Comments: To be published in Science. 12...
  15. wolram

    What is the Explanation for the Cosmic Microwave Background Radiation?

    http://www.physics.ubc.ca/~heyl/ns2005/prospectus.html Neutron Stars at the Crossroads of Fundamental Physics I. Organizers Jeremy Heyl – University of British Columbia Vicky Kaspi – McGill University Feryal Özel – University of Arizona Krishna Rajagopal – Massachusetts Institute...
  16. B

    Main Sequence Stars: Mass & Beyond

    Straight forward questions that's been bugging me a little. Why do most stars lie on the main sequence whilst others don't? Is it just purely characterised by the mass? Thanks
  17. N

    Where can I find PDF Black holes White Dwars and Neutron Stars

    Where can I find PDF "Black holes White Dwars and Neutron Stars" Gents, Could u pls advise me if you know where can i find book "Black holes White Dwars and Neutron Stars" Authors Shapiro, Tuekolsky (free PDF or DJVu or other format) Thks
  18. J

    Form and Features of Asteroids and Neutron Stars

    What are the forces that give asteroids their shapes? And what shape do neutron stars that do not rotate around its axis have?
  19. T

    Habitable Worlds circling non-Sol like stars

    Is it possible, say, for a world like Earth (or, at least enough like Earth that people could live there without space suits, etc.) to orbit a Red Giant star? What would such a world be like? \phi The Rev
  20. marcus

    Neutron stars: good short article

    one reason it's good is that it is written for the Wiley "Encyclopedia of Physics" http://arxiv.org/abs/physics/0503245 Neutron Stars Gordon Baym, Frederick K. Lamb Comments: Encyclopedia of Physics 3rd ed., R.G. Lerner and G.L. Trigg, eds., Wiley-VCH, Berlin Abstract: "This short...
  21. J

    What are the orbital parameters of a planet orbiting a star?

    Here is my problem: The star is 59 light years from the Earth and has a mass of 1.90×1030 kg. A. A large planet of mass 1.20×1028 kg is known to orbit this star. The planet is attracted to the star by a force of 3.50×1026 N when their centers are separated by a distance equal to the...
  22. G

    Are Solar Systems Common in the Universe?

    AKA solar systems rare?
  23. A

    Calculate Density of 3 Stars: White Dwarf, Neutron Star & Black Hole

    Alright, I just need to find the density of three stars in g/cm^3. I have been given the following information: Density of a star = Mass / ((4/3)*pi*Radius^3) First star is a white dwarf: Mass = 2 * 10^30 kg Radius = 5000 km Second star is a neutron star: Mass = 2(2 * 10^30) kg...
  24. B

    First Stars - How big - Now Black Holes?

    First star-forming gas clouds were much denser. (Early Universe was smaller and prior to stellar fusion also had more hydrogen.) Did not most first generation stars leave Black Holes behind when the rapidly completed their life cycle? How much did the universe expanded while the first stars ran...
  25. Loren Booda

    Dark matter galaxies, stars and planetoids

    Can and do they exist primarily separate from ordinary matter, and how might they be detected (e. g., gravitational lensing, Newtonian mechanics)?
  26. Q

    Are there still stars being discovered?

    Are there still stars being discovered? I mean are there stars whose light has not reached us. I think that is only possible if after the BIG BANG matter traveled away from each other with more speed than the speed of light.
  27. E

    Is Teegarden's star really that close?

    Could it be that all the stars we see in the sky no longer exist?
  28. F

    How is the Distance Between Stars Measured?

    The distance between stars is measured by the time it takes light to travel between them correct? How do you measure the time it takes? for example from CNN "It is 12.7 billion light-years away, meaning the light just recorded -- by NASA's Chandra X-ray Observatory -- took 12.7 billion...
  29. E

    What color would the end star be if a red star and a blue star collided?

    Sooner or later its bound to happen. Has there been a recent observed colliding force between two stars in our Galaxy? i'm keen to see a collision between a red star and a blue one to know what colour the end star will be. E
  30. G

    How Can You Measure Star Distances Using Ancient Astronomical Techniques?

    Hello, Well let me give a pithy description of what’s going on. There are 6 fixed stars (constellation) within a building and an observatory, which is the only place measurements of distances and positions can be calculated. Now I need to find the distance to each start using parallax methods...
  31. Loren Booda

    A body intermediate to stars and galaxies

    Out of the primordial hydrogen, what were the largest star and smallest galaxy that eventually formed?
  32. marcus

    The Upper Limit of Star Mass: Theories and Uncertainties | arXiv.org"

    I was reading about the greatest mass that stars can have http://arxiv.org/astro-ph/0501135 it is not 100 percent sure but 200 solar masses looks like an upper bound on the mass it is also not certain why. if there is some upper limit, say between 130 and 200 solar mass, then WHY...
  33. N

    Doppler effect and color of stars

    I was reading somewhere that the because of the red shift it can be concluded that the galaxies are moving away from us. But still when we talk about stars, red color refers to cooler stars rather than stars moving away from us. Why?
  34. Chronos

    Astronomers reveal biggest stars yet seen

    Another interesting release: Astronomers reveal biggest stars yet seen http://www.newscientist.com/article.ns?id=dn5046 It just keeps getting weirder when you look far enough back. Modern theory is sometimes frighteningly accurate.
  35. D

    What is the concept of concentrated mass?

    A little confused, could someone explain what it means when it is said that "the point in a body or a system of bodies, at which the total mass of the body or system may be regarded as concentrated".(definition of Center of mass) What does it mean concentrated? I'm trying to understand the...
  36. B

    Exploring Distant Stars and Photons with Hubble Telescope

    suppose we look to distant galaxies or stars tru a telescope like hubble. We can see an image of the star becouse it's emiting photons and this photons are reaching the Hubble telescope... Knowing the size of the universe, the size of the star, the size of the Hubble telescope, and it's...
  37. D

    What is the allure of rock stars?

    As I type this, I listen to “The Zephyr Song” by Red Hot Chili Peppers, and I have just begun to contemplate odd ideas. I don’t understand why I feel some sort of awkward attraction for the life of rock stars -- especially eccentric, intelligent, and such wild ones. I am a practicing Christian...
  38. L

    Learn the Night Sky: Star Placement & Familiarization

    Does anyone know a good web page for becoming familiar with the night sky... and where certain stars should be etc..?
  39. J

    Can planets exist around twin or triple-star systems?

    It seems that planets can not exists around twin-stars or even triple-stars. I mean that the planet orbits both stars, and not just one of the two/three. Why is that? The planet can just circle around the centre of gravity, what is so special about a twin or triple-star system?
  40. marcus

    Movie of stars orbiting a black hole

    http://www.mpe.mpg.de/ir/GC/images/movie2003.mpg four stars photographed (timelapse over years) doing various type orbits around the million-solar-mass hole at the center of Milky one doing elliptical, one kind of long skinny parabolic, one on what looks like a one-shot hyperbolic...
  41. Orion1

    Neutron stars and colour force

    Einstein field equation gravitational potential: \nabla^2 \phi = 4 \pi G \left( \rho + \frac{3P}{c^2} \right) General Relativity gravitational pressure: P_e = \frac{c^2}{3} \left( \frac{\nabla^2 \phi}{4 \pi G} - \rho \right) Classical Yukawa Pressure: P_y = f^2 \frac{e^{-...
  42. M

    Mysterious Green Stars in Patriotic Photos

    in some photos, mixed among the red white and blue stars, are green stars. Any idea what causes this?
  43. N

    Measuring the distance of stars

    Is the parallax method still the one most widely used (for the relatively nearer stars at least)? More particularly, is it still the case that we take one measurement then wait six months until the Earth is on the other side of the sun and take a second measurement? Or has this been superseded...
  44. K

    Neutron stars and colour force

    If neutrons stay intact and get closer together than 10^-15 metres in a neutron star, would the exchange of mesons between neutrons stop and be replaced by the exchange of gluons, and would the gluons cause an attractive or repulsive force between neutrons? A repulsive force could stop the...
  45. N

    Why do stars emit electromagnetic waves?

    What process within a star causes it to emit electromagnetic waves (light, x-rays etc.)? What is the source of the electricity / magnetism inside the star?
  46. S

    Flares etc: why do they only exist on stars?

    Why can't the Earth's magnetic field go bazooka! It's made of iron and it's got poles...
  47. T

    The Colors of Stars: Why Are Some Red and Others White or Blue?

    Hi, i read something about light turning red due leaving gravitational fields. A long time ago i played a game called ascendancy which was about colonizing solarsystems and battling other species and they had a few different kinds of stars in it, a few of these where white and blue dwarfs and...
  48. Antonio Lao

    Exploring the Cause of Star Explosions

    Why do Some Stars Explode ? It is a fact that stars do explode. But why? Cosmological theory says it is caused by the acquisition of mass beyond the Chandrasekhar limit. My hypothesis is that explosion is related to an intrinsic metric variable. This variable is the parameter for all possible...
  49. R

    Why do neutron stars have magnetic fields

    An article by W Tucker and K Tucker at NASA says that neutron stars have magnetic fields. If a magnetic field is created by moving charges, and neutron stars have not net charges to move, how are the fields created?
  50. A

    Are There Stars with Highly Eccentric Orbits Around the Center of Their Galaxy?

    An interesting question has arised from one of the discussions around here: are there any stars that have a highly eccentric elliptical (or even parabolic/hyperbolic) orbit around the center of their gallaxy, somewhat like comets have in our solar system? And for that matter, are there any stars...
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