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. V

    Astronomy: Explaining Why Stars on Top Left of Main Sequence Spend Less Time

    Hi, I was just wondering if anyone could help me with this question. I couldn't find anything on the internet. Q12 Explain why a star on the top left of the main sequence will spend much less time on the main sequence than another star on the lower right
  2. J

    G Magnitude to V magnitude stars in a CCD

    I have astronomical images taken using a CCD camera and telescope using a g filter. In the images I have stars with what is called instrumental magnitudes (from the photon count in each pixel). My question is if there is a way to change this g instrumental magnitude to a real V magnitude. I...
  3. A

    Why don't black holes look like neutron stars to outside observer?

    Yesterday, I read about Hawking's new proposal regarding the firewall paradox. A more general thought about standard black holes occurred to me. Black holes including stellar black holes are of course always presented as if the event horizon is an invisible barrier, which the unfortunate...
  4. marcus

    Planck Stars: Carlo Rovelli & Francesca Vidotto

    I just logged this on the biblio thread. This is in case there are questions, or things people want to discuss. http://arxiv.org/abs/1401.6562 Planck stars Carlo Rovelli, Francesca Vidotto (Submitted on 25 Jan 2014) A star that collapses gravitationally can reach a further stage of its...
  5. T

    Do less bright stars have redder spectrum?

    Do less bright stars have redder spectrum?
  6. rrw4rusty

    All stars in Milky Way equal one square light year?

    Hello, Can anyone confirm or, refute and correct the following statements? The volume of our galaxy, is roughly 8 trillion cubic light years. The combined volume of all the stars in our galaxy only equals one cubic light year. Thanks for any help you can provide! Rusty
  7. Chronos

    First detection of pop III stars?

    This paper, http://arxiv.org/abs/1312.5320, discusses the possible detection of galaxies almost entirely consisting of pop III stars.
  8. I

    Neutron Stars vs Atoms: A Comparison

    So how much denser than is a neutron star than an atom? Is it called a neutron star because electron's orbital effect no longer exists/acts the same, so as to no longer cause 99.99% of everything to be space?
  9. Chronos

    Prevalence of Earth-size planets orbiting Sun-like stars

    This paper, http://arxiv.org/abs/1311.6806, discusses Kepler mission results. The authors deduce that 5.7% of sun-like stars have an Earth sized planet orbiting in the habitable zone. That is a shockingly large number, IMO. Considering there are an estimated 26384 class G class stars within 324...
  10. M

    Gravity, center of planets, stars

    Hello , it is said that at the center of planets and stars there is zero gravity because , assuming spherical geometry, the gravity from all sides comes together and cancels out in the middle and everything that is in close approximation to it. Now that said I also read that the pressure at sun...
  11. 2

    Estimate the number of stars with present-day luminosity function

    Folks, Please don't flame me for this as it is probably already answered somewhere. How does one estimate the number of stars in a 500pc^3 which have a range of 10^-2 to 0.1 Lsun? I am not a maths head, so simple answer that would have the formula would be great. Thanks
  12. L

    Period of revolution of two double stars

    Homework Statement Two double stars of the same mass as the sun rotate about their common center of mass. Their separation is 4 light years. What is their period of revolution? Homework Equations Lagranian = T - U = \mu\dot{r}^{2}/2 + \vec{L}^{2}/2\mu r^{2} - Gm_{1}m_{2}/r F = ma =...
  13. P

    Neutron stars on collision course.

    Simply put, either by chance or a suicidal type-III Civilization, 2 neutron stars, or a neutron star an a large-ish main sequence star are thrown at each other and fall STRAIGHT into each other. What happens? Does the ~1.4 C Newtonian impact speed result in them getting blown up and releasing...
  14. Superposed_Cat

    How are neuton stars intensly magnetic?

    Neutrons are neutral (obviously). I thought only charged particles could be magnetic. So how are neutron stars magnetic? Thanks for any help.
  15. A

    Astronomy homework question - stars emitting in infrared

    Astronomy homework question -- stars emitting in infrared Dear All, Hello! This is my first post. I am taking a distribution introductory astronomy course (low on math) and am having trouble with this question: Which object emits more intense infrared radiation: _ the Earth _ the Sun...
  16. S

    Why Do Stars Appear to Move in a Four-Year Cycle?

    Hello everybody! I read that because of about 4 min difference between sidereal day and solar day, at the end of a year each star has rotated 366.25 times(apparently). I looked up in software such as Starrynight, the position of for example Sirius at 1 September of 2013, 2014, 2015 and so on. I...
  17. L

    Why Aren't the Stars Visible at Night?

    Isn't the reason the sky isn't bright at night ,due to all the stars, because the fact the stars aren't bright enough to be seen by the naked eye after a certain distance? So it doesn't necessarily depend on the fact the universe doesn't have to be infinite or expanding? I know the intensity of...
  18. B

    Research topics in Variable stars

    Hey, I am an undergraduate physics & astronomy student. I am looking for a good research topic to work on for this semester. I have access to a .5 m telescope in Louisville, Ky (38 deg N), a .5 m telescope, and possibly another with a built in spectroscopy as well as photometry capabilities...
  19. C

    Speed light from distant stars

    Light from distant stars is doppler shifted. Is the measured speed of light from these stars lower than the speed of light from the sun? Would one expect the permativitity of free space to vary over the universe?
  20. R

    Exploring Quark Stars: Facts and Mysteries

    Good article about Quark stars: http://www.nanowerk.com/news2/space/newsid=31543.php of course, the title instantly makes me think, "Piccard would not approve!" :biggrin:
  21. O

    Special relativity and stars' masses

    Hello everybody I am a first year student and I have a question about stars' masses. I've been thinking about, what happens to a star's mass cause of the high speeds of the particles there. Do we consider the 'extra' mass, 'caused' by relativity (really high speed particles are heavier than...
  22. B

    Black Holes & Stars - What Do We Know?

    Hi, One can register waves of light. Is there anything known different then stars where we receive information from? Say black holes? I really guess.
  23. T

    Do Stars in Space Interact Through Wave-Cancellation?

    I've been comparing light as particles and waves in outer space when it occurred to me, if you throw two rocks of equal weight into a pool side by side the waves created will cancel out. Do stars In space effect each other in this way?
  24. F

    Brown Dwarf more massive than stars?

    Is it possible for some brown dwarves to be more massive than some stars?
  25. S

    Functions for properties of stars and their planets

    I'm working on a video game set in space, and I'd like my portrayal of the galaxy to be as scientifically accurate as is feasible. I've been looking into metallicity functions, but I can't make any sense of what I'm seeing, and I don't even know where to start on the subject of algorithms for...
  26. U

    Travelling towards distant stars

    It is often quoted that, when we see stars from Earth through, for example, a telescope, we are not seeing them as they are now, but how they were millions of years ago (because the travel of light is not instantaneous). However, say if I take a picture of a star from Earth, then jump into a...
  27. adjacent

    Stars Galaxies in the universe

    If there are so many galaxies in the universe,Why don't wee see a night sky with full of light(from the galaxies or stars)
  28. N

    Why do stars appear to have different colors?

    Obviously if you looked at one it would appear white as it's so bright and stars are black bodies so emit all over the spectrum (correct me if I'm wrong). But as the sun is a yellow star, it looks yellow around it (unless that's the atmosphere of Earth getting rid of blue?) and has a yellow...
  29. R

    Finding period of revolution of stars?

    Homework Statement In a certain binary-star system, each star has the same mass which is 7.5 times of that of the Sun, and they revolve about their center of mass. The distance between them is the 7.5 times the distance between Earth and the Sun. What is their period of revolution in years...
  30. O

    This whole we're made from stars thing.

    This whole "we're made from stars" thing. I keep coming across internet memes that say all the elements (except H and He) were created from stars, and therefore, humans are the product of stars. Every atom in our body came from a star billions of years ago. I never gave it a second thought...
  31. P

    Why do stars collapse in ot itself? Part 2

    I think space-time exerts a pressure on substance and it makes star to transorm black hole.I think this can be possible becuse we know that space-time is something which it can be tiltled and it can creates virtual matters.Space-time is real thing.So Can be space-time exert pressure on substance ?
  32. A

    Why don't black holes become stars again?

    1st post here - new to the forums - sorry if this is a stupid question.I was thinking, if the universe is approximately 98% hydrogen, you would think the matter a black hole consumes would be mostly hydrogen. So you have a black hole, it's consuming a ton of hydrogen, squeezing and condensing...
  33. P

    Why do stars collapse in on itself ?

    Phase of the star's death ,when star's fuel ended,we know that it will collapse into itself Why do stars collapse in on itself ? How to collapse in on itself ? Although the end of the fuel in the star,how to collapse in on itself? Because inside the star there is no...
  34. R

    Neutron Stars: Quantum or Classical Objects?

    Dear all, I'm starting to study neutron star accretion and a lot of physical phoenomena coming from different scales come along and a consistent picture is frankly hard to grasp. But for now, a real mind boggling question can't exit my mind. Are Neutron stars, as a whole Quantum, or...
  35. 4

    Stars moving faster than the speed of light

    If two stars, each moving away from each other faster than the speed of light, how would the light behave? Let me add more detail, if you have two stars a light-year away from each other and "off" and then accelerated both of them in opposite directions traveling at say .60c and identically...
  36. S

    Can we expect stars to become visible only in infrared after a while?

    I wonder if some old stars will become invisible to our eyes after a while because of the redshift? Since they get further away from earth, maybe we won't receive their radiation in visible light any more? I'm talking about stars that are not obscured by any other objects or dust, which you...
  37. I

    Conceptual Question: Stars in intergalactic space with speed

    Homework Statement http://postimage.org/image/7rgmo1527/ How is the speed decreasing and acceleration decreasing in this problem? Homework Equations Kepler's Laws? The Attempt at a Solution I remember in lecture my professor was saying something along the lines of objects...
  38. T

    Relative population density of stars and galaxies

    Homework Statement The number of stars around the Sun, within a distance of 17 ly, is 75. The number of galaxies in the local group, within a distance of 4.0 x106 ly from the Sun, is 26. Calculate the average popluation density, per ly3, of stars and galaxies. Homework EquationsThe Attempt...
  39. R

    Percentage of stars that become black holes

    What percentage of stars become black holes? I tried to find this information through Google searching and the only thing I could find was a statement that said less that 1 in 1,000 stars in our Milky Way galaxy have enough mAss to become a black hole.
  40. J

    Probability of stars in a multiverse

    Having recently read two books (Just Six Numbers by Martin Rees and The Fallacy of Fine-Tuning by Victor Stenger) I am confused on one issue. Assuming a multiverse, what is the probability that stars and galaxies would form in a universe. Stenger would say it is high but Rees would say it is...
  41. N

    Deriving the number of stars as a function of flux and bolometric magnitude.

    QUESTION A star of bolometric luminosity L at a distance r will exhibit a bolometric energy flux F given by F = L/4πr2 in the absence of obscurity. A. Assume that all stars have the same bolometric luminosity L and that stars are uniformly distributed in space with a number density n...
  42. O

    Binary stars, time taken to collide

    Homework Statement A binary system consists of two stars of equal mass m orbiting each other in a circular orbit under the influence of gravitational forces. The period of the orbit is τ . At t = 0, the motion is stopped and the stars are allowed to fall towards each other. After what time t...
  43. A

    Rate of planets flowing into stars: Discussion

    I just posted a question to ask for help on the Calculus & Analysis section to determine how fast planets go into stars. I should give a little background, in addition to pointing you to my latest two short papers: http://arxiv.org/abs/1301.4229 and http://arxiv.org/abs/1211.1984 I have...
  44. A

    How fast are planets flowing into stars? I got the equations.

    I am an astronomer with a research question: I want to evolve this equation for planet distribution: \frac{{\operatorname{d}}f(P)}{{\operatorname{d} \operatorname{log}}{P}}= k_P P^\beta \left(1-e^{-(P/P_0)^\gamma }\right) as a function of period ``P'', by using an equation for change of P. I...
  45. alemsalem

    Measuring the distance to stars using parallax

    how can you use the angle to determine the distance if you don't know the motion of the star itself. is it by observing the star from the same point in two different years and then subtracting off this effect?
  46. samsara15

    Redshift & Far Distant Stars: Effects on Temperature

    Wouldn't the Redshift result in decreased heat from far distant stars, resulting in an equilibirum temperature, much lower than the average star?
  47. E

    What can we say about neutron stars?

    Homework Statement About neutron stars we can say that: a) They are newborn stars b) They are generated when a Solar-type star dies c) They are generated when a very massive star dies d) They are in the main sequence Homework Equations None. The Attempt at a Solution I'm...
  48. G

    How to Calculate Luminosity of an RR Lyrae Star?

    Homework Statement Given pulsation period in days, (surface temperature) and mass of an RR Lyra variable star. How do you find luminosity? Homework Equations The relevant equations seem to be for finding the luminosity of Cepheids... there are no equations for Lyrae... Other than, absolute...
  49. S

    Stars heat up when they lose energy

    Hi There Just came across the idea that stars heat up when they lose energy and have been thinking about why this might be... below is my best guess: Stars heat up when they lose energy because this implies that the ions that compose the star each experience a reduction in their kinetic...
  50. P

    Distribution of stars by Metallicity and Redshift

    Is there a large database that contains, for many stars, their estimated metallicities as well as their redshifts (or, equivalently, distances from us) and masses? What I'm trying to do is estimate the chemical abundances at different ages of the universe. If I have the metallicity of a star...
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