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. alemsalem

    How will the pattern of stars in the milky way change over time?

    how will the pattern of stars in the milky way change as the galaxy rotates, will they be rotated in some direction or drifted or something? and what is the timescale for that to happen? Thanks!
  2. Islam Hassan

    Hydrogen Fusion Cycle in Stars: Storage/Transport/Consumption?

    I have always been wondering how in an active star the H/He fusion cycle is "managed" for lack of a better word, a little like a logistical analogy really. Specifically: i) Where in the star is the yet-to-be-fused H 'stored'? ii) Where in the star is the He result of fusion 'stored'...
  3. D

    Elemental Abundance in Stars - The Curve of Growth

    I am studying for my undergraduate Astrophysics module and the lectures notes say that that all abundances are measured relative, in terms of H = 12.00 by mass or number of atoms. Is this correct? I thought it was all based off of Carbon = 12. Am I missing something?
  4. X

    Why Does the Sky Appear Blue? Why Can Stars See Us?

    Hello, I have a novice question. I know that the sky color is blue (at noon, when there is no clouds), because the air particles (mostly N2 and a little O2 ...and other) scatter short-wavelength light more than longer wavelengths. That is why we can't see the stars trought the atmosphere...
  5. E

    Electron influence on nuclear fusion in stars.

    My latest thoughts about the life, universe and everything has brought me to this scenario: [in a stars core, but a perfect thought situation, for the sake of illustration] there is a proton, electron and proton on one line in this particular sequence. I think, that the electron, witch has...
  6. M

    Calculating Binding Energy of Neutron Stars

    in general how to calculate binding energy of neutron star? in simple details, what tricks may be included in such a problem..
  7. Drakkith

    Population III Stars Metallicity

    Early Population III stars in the universe were very metal poor if not metal free and were apparently required to be much larger than todays stars, up to hundreds of solar masses, to form. What does the metallicity have to do with this?
  8. Philosophaie

    Stars of Constellations exist in what Spiral Arm of the Milky Way Galaxy?

    The Milky Way is a Barred Spiral Galaxy 100,00 -120,000 Across. I was wondering if there is a website or list of what Stars of Constellations exist in what Spiral Arm. I there exists: Orion-Cygnus Arm Perseus Arm Scutum-Centaurus Arm Norma and Outer Arm Carina-Sagitarius Arm...
  9. V

    Classsification of Pre main sequence(PMS) stars

    Hello everyone, there is a classification of PMS based on color-color diagram as class 0, class I and class II. can some one please explain more about this classification or provide me with some useful link. I am trying to plot color-color diagram of an Open cluster using Wise Band data.
  10. T

    Where can I find a downloadable database of stars?

    Stars database download? Stars database download? I found "NStars", it sounds like they have what I'm after, but the site is not online as is currently undergoing major redesign. Is there any other place? I'm looking for coordinates data, size, spectrum, luminosity and such. Maybe some free...
  11. S

    Stars Accelerating towards each other

    Homework Statement Two stars move in circular orbits about one another with period T due to their mutual gravitational attraction. If the objects are suddenly stopped, show that they will then collide with one another after a time T /(4√2) Homework Equations T = ∫dt dA/dt = (1/2)r2d∅/dt Fg=...
  12. G

    What caused two stars to move in a zig zag pattern and disappear?

    Few years ago while traveling to Bosnia, my cousin and I looked at the sky. We were in a very small village where almost no lights disrupted the visibility of the stars. While looking we noticed that there were two stars which were a bit brighter, almost like the north star, except that the...
  13. L

    How Do Planets Orbit Stars in Elliptical Paths?

    Homework Statement A planet orbits a star of mass M = (3*10^30)kg in an elliptical orbit. The planet is r_1 = (0.8*10^11)m from the star at its closest approach (periastron), and r_2 = (1.6*10^11)m at its furthest (apastron). (G = (6.67*10^-11) Nm^2kg^-2) (a) Calculate the semi-major...
  14. T

    Searching for Colliding Stars: Has the Phenomena Ever Been Observed?

    Are there any known examples of colliding stars? has the phenomena ever been observed? tex
  15. P

    Can I see stars through the moon?

    Hey folks, A couple of weeks ago I took a photo of the moon, not sure if you will be able to see this in the image, but if you zoom in a bit after downloading it, you can see what I think are stars all over the background... ...The reason I say think is that I can see them through the...
  16. B

    Radial pulsation of stars problem

    I am trying to do the problems in this set: http://farside.ph.utexas.edu/teaching/336k/lectures/node13.html They seem quite hard, but I think I've managed to get to #4. The statement of the problem is at Problem 4 in the above link. My attempted solution is the following: The...
  17. D

    What is the period of rotation for three stars forming an equilateral triangle?

    Homework Statement Three stars, each with the mass of our sun (1.9891E31 kg), form an equilateral triangle with sides long. The triangle has to rotate, because otherwise the stars would crash together in the center. What is the period of rotation? Homework Equations F1=GM2/r2 ƩF=0...
  18. E

    Understanding Spectra Shifts of Distant Stars

    We can know about distant stars temperature,its elements etc by observing its spectra... But that star has a velocity related to us... The spectra will be shifted... Then how do we know the correct spectra? Another ques... Suppose, something emits a X ray... Then will its characteristic will...
  19. S

    Estimate the Hydrogren Burning Lifetimes of Stars

    Homework Statement Estimate the hydrogen burning lifetimes of stars on the lower and upper ends of the main sequence. Lower end: M = 0.085 solar masses; log10(Teff/K) = 3.438; log10(L/Lsolar) = -3.279 Upper end: M = 90 solar masses; log10(Teff/K) = 4.722; log10(L/Lsolar) = 6.045...
  20. Drakkith

    Role of Strong Force in Neutron Stars

    Does the strong force have any major role in neutrons stars other than obviously holding individual nuclei together? Would low energy neutrons tend to "clump" together in the core?
  21. A

    I have a problem where two stars are at a certain time r0 apart from

    I have a problem where two stars are at a certain time r0 apart from each other. They both has the mass of m, and at that time one of them is at rest and the other has the speed of v0 in the opposite direction of the gravity force that the other star exerts upon him. I need to find the maximal...
  22. K

    Stars or Transistors: Which Outnumbers the Other in Our Universe?

    This thing just popped into my head last night while describing to a friend, possible end of our universe. Which one is higher? the number of stars in our universe or the number of MOSFETs we mass-fabricated in our civilization ? My guess is though number of stars. What do you think...
  23. L

    Source of magnetic field of Neutron stars.

    If Neutron stars are what they are... I mean made of literally only neutrons ...how do they generate such immense magnetic field (of the order 10^8 or something) .. I think the name is somewhat misleading and it ought to have a near half ratio of protons ...But still being so compact how...
  24. gmax137

    Do You Know Why Stars Have Different Colors?

    In the Letters section of the August issue of Physics Today, a reader claims that when he asks 'newly minted physics PhDs' to explain why stars have different colors, that 75% of them answer incorrectly. How would you answer the question?
  25. A

    Can you have binary stars with the same radius but different masses?

    The title speaks for itself. I'm just curious and I couldn't find anything on it. Particularly, what if they are a close binary (period less than 50 days)?
  26. 1

    Does entropy decrease during the formation of stars and the freezing of water?

    Does entropy decrease when stars are created from a dust? Does etropy decrease when water freezes?
  27. R

    Gravitational Waves & Neutron Stars/Black Holes: Observed Velocity Decrease

    2 closely orbiting massive objects are predicted to deserve the fabric of spacetime so much that they will release a ripple of gravitational waves. When geavitational waves are released, they slowly loose velocity energy and spiral into one another. 2 pulsars were discovered orbiting each other...
  28. I

    Could Dark Matter Explain the Mystery of Shear Viscosity in Neutron Stars?

    The situation is this: In a neutron star the Coriolis force induces Rossby waves, just like on Earth. These are waves with very long wavelength -- like halfway around the Earth -- and very large volume but very little amplitude, like fifty meters. On Earth they have a big effect on climate...
  29. B

    Stars: North, South & Equator Positions

    Would people on the north pole see a totally different set of stars than people standing on the south pole..? Would the Big Dipper be visible from the South Pole..? Would people living on the equator see even a different set of stars..?
  30. KenJackson

    Positions of stars as seen from Mars-different than from Earth?

    Positions of stars as seen from Mars--different than from Earth? I'm reading a novel that says , "On Mars the stars seemed a bit out of place, not in their correct position[s]." Could that be true? I suspect that if you disregard anything within our solar system, the stars would be so far...
  31. J

    Question about stars and light after a trip to the Hayden Planetarium

    Ok so I was at the Hayden Planetarium and a plaque was describing a star billions of light years away that we can see because it is so much brighter. I was puzzled thinking about this. If the speed of light is constant there is no reason the intensity of it's light should affect how far away we...
  32. K

    Gravity to surface and speed of stars

    the law of gravity states that objects accelerate at a constant speed, but I'm wondering that if you take into consideration that time travels slower the closer you are to the surface of the earth. doesn't that imply that although it seems to us that it is constant acceleration that, it is...
  33. D

    Why Can't We See Stars Beyond Earth?

    How come in pictures taken from space, we cannot see stars or the sun? Is it the atmosphere that allows us to see them? Click on image to make bigger
  34. T

    How Do You Calculate Angular Separation of Stars Using Different Coordinates?

    Hi all. I have been looking for a formula that gives me the angular separation of two stars. So far I just found this one :http://www.skythisweek.info/angsep.pdf but it uses the RA and DEC of the 2 stars, and I need to use the hour angle and DEC. Is it the same thing? Do I just replace HA by...
  35. R

    Do scientists know why planets orbit stars on the same dimentional plane?

    Pretty straight forward. The same generally holds true with moons belonging to the same planet. If we don't have concrete reason why this occurs, are their any strong theories?
  36. A

    Could a Star Collapse into a Quark Star Instead of a Black Hole?

    Hi, In the case of the gravitational collapse of a star where neither electron or neutron degeneracy pressure is sufficient to prevent further collapse, is there a possibility that the star will become a quark star rather than a black hole? From what I understand, quarks are perpetually...
  37. D

    What if Neutron Stars Collide: Matter, Mass & Metal

    what if...Neutron stars I know likely its been answered but if two neutron stars collid, it wouldn't seem that there combined massess would forum a black hole...if I recall that any matter striking the surface of a neutron star impacts with such violence that there is a massive explosion. So I...
  38. B

    How fast are stars orbit speed?

    1. How fast are stars orbit speed in elliptical galaxies (from and to) 2. How big differences are there typical between Perigalacticon and Apogalacticon in a. elliptical galaxies and b. spiral galaxies?
  39. D

    Will Neutron Stars Last Forever Without Proton Decay?

    If proton decay does not occur, will neutron stars just last for eternity unless something collides with them?
  40. E

    Big doubt in expansion of stars

    To a close time, i settled my interpretation of expansion of the SUn... When the H=fuel in the core is over, the helium atoms is now having more density. Hence, the pressure increases on the outer H-fuel which ultimately will fuse faster and faster making a great deal of energy casuing...
  41. Q

    Light, Wavelength, Space, and the color of Stars

    Question 1: http://i.minus.com/iOmQvqhasiXmx.png My reasoning is curves 3 and 4. These curves have the biggest relative brightness of orange. Question 2: Stars are called blackbody radiators because they a) never appear perfectly black b) always appear perfectly white c)...
  42. I

    Do Neutron Stars Eventually Decay Into Black Holes?

    After reading "The Five Ages of the Universe" by Greg Laughlin and Fred Adams, I wondered, if all matter composed of ordinary atoms (protons decay) decay, and black holes decay due to Hawking Radiation, do neutron stars decay in any way? They are composed entirely of neutrons that are kept...
  43. mrspeedybob

    Dim Stars: Are There Almost-Black Holes in the Universe?

    If there were a star which was almost, but not quite massive enough to become a black hole it seems as though gravitational time dilation should make it appear very dim. If time dilation resulted in 1/1000 of the time passing within the star as passes for us, we should see a star emitting 1/1000...
  44. T

    Are All Stars the Same Size Due to Nuclear Fusion?

    Why are all stars not the same size? Given that the nuclear fusion within a star begins when the pressure/temperature in the core of a collapsing cloud reaches certain parameters, why are all stars not the same size?
  45. R

    Stargazing What creates the star points in telescope pictures of stars?

    what creates the "star points" in telescope pictures of stars? it's a basic question. e.g. see this hi-rez Hubble Deep Space pic: http://upload.wikimedia.org/wikipedia/commons/9/9b/Hs-2004-07-a-full_jpgNR.jpg i count about three bright objects with pointy cross-like projections from...
  46. E

    Classification of stars by spectrum

    This topic is quite confusing.. In the first place, how can we classify for example all O class stars would be having the same spectrum with the same absorbtion lines... Secondly, i just need a clarification on what do these absorbtion line reprsent and upon that how can we decide whether a star...
  47. E

    The stellar evolution. Low Mass stars

    Well, this topic is driving me crazy so any answers or replies should preferably be detailed and in an easy way.. My question is i want a sum up of the evolution of a low mass star... I will give my answer and tell me where are the dropouts.. Let me begin,With a protostar accreting hydrogen in...
  48. E

    Spectrum of Stars: Explaining the Spectral Lines of Stars

    I've been deep into this topic for a couple of days and it is becoming just more and more confusing. I am yet a beginner so i would like explanations to be detailed and easy. Moving directly, my doubt is.. We treat stars as black bodies which emit radiation... From where did the stars originally...
  49. E

    Exploring the Evolution of Low Mass Stars

    In my book, it is written that in an evolution of a star, specifically the sun, IT will run low on hydrogen fuel but will have produced so much energy that it will expand slightly... Once H- fuel is used, the star will start fusing helium nuclei. This complex process can cause an explosion that...
  50. D

    Distance of planets from stars and revolution

    Hi dudes, My questions are: What does planet's revolution depends? If for example is there a planet 4 time the mass of the Earth at about its same distance from a star 4 time the mass of sun, could it be it has the same revolution period ot the eath?
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