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. J

    B Explaining the Color Variations of Main Sequence Stars

    If all main sequence stars are engaged only in hydrogen fusion, why don't they all exhibit the same color?
  2. moenste

    Two stars rotating about a centre of mass -- Find speed

    Homework Statement The diagram shows a binary star system consisting of two stars each of mass 4 * 1030 kg separated by 2 * 1011 m. The stars rotate about the centre of mass of the system. (a) (i) Copy the diagram and, on your diagram, label with a letter L a point where the gravitational...
  3. A

    B Gravitational waves 50X total stars and not be felt?

    Hi all, On the subject of being able to feel gravity waves as a human without machines, I couldn't find a very definitive answer to what I was looking for. First, I reference the video: at 40 tp 55 seconds. Its called: LIGO, journey of a G wave. They say in this clip that when the black...
  4. Borg

    Stargazing GAIA - 1 billion stars and counting

    The GAIA telescope has been mapping stars in the Milky Way with unprecedented quality and quantities. It has been assembling the most detailed 3D map ever made of our Milky Way galaxy and has currently mapped over 1 billion stars. There are already hints that the Milky Way may be shaped...
  5. Irfan Nafi

    B What kind of supernova do binary neutron stars produce?

    I was wondering if a binary system of neutron stars were to collide what kind of nova or supernova would it be. Is it different if the final mass of the neutron star is sufficient enough for a black hole or if it still a neutron star?
  6. wolram

    B When Did the First Stars in the Universe Form?

    http://arxiv.org/pdf/1609.00716.pdf Date: September 2, 2016 Source: European Space Agency (ESA) Summary: ESA's Planck satellite has revealed that the first stars in the Universe started forming later than previous observations of the Cosmic Microwave Background indicated. This new analysis also...
  7. Irfan Nafi

    B How do stars orbit the center of the Milky Way?

    I've been wondering how all the stars of the Milky Way orbit the center of the Milky Way almost like it orbits Sagittarius A*. It is possible that there is a common center of mass that happens to be in the center of the Milky Way that also happens to be in the center, ultimately giving it its...
  8. J

    B Do Atoms in Neutron Stars Touch Each Other?

    I have heard that in neutron stars, the atoms do not repel nor do they attract. In a sense, could this mean that these atoms nucleuses could touch each other, or could the atoms touch in general?
  9. J

    Apparent Flux and number of stars

    The stars in our Galaxy have luminosities ranging from $L_{\text{min}}$ to $L_{\text{max}}$. Suppose that the number of stars per unit volume with luminosities in the range of $L$, $L+dL$ is $n(L)dL$. The total number of stars per unit volume if clearly $$n = \int_{L_{min}}^{L_{max}} n(L)dL.$$...
  10. Phil A

    Stargazing Stars in sky given arbitrarily sized/oriented window

    Hi All, I've been taking a bit of a crash course in astronomy for a raspberry pi project I'm toying with. I basically want to create a window to the night sky without the actual window but a screen to replace it. Barring the practicality (psh, who needs that) of doing this, it's been a lot of...
  11. sophiecentaur

    Stargazing Artificial Stars for testing telescopes: the final answer?

    I feel the need to make sure my Dobsonian scope is working at its best. I was considering a conventional (?) red LED collimator but I suddenly begin to see these Artificial Stars on sale. Would that be a better way to go? Anyone have experience of them? Looking at real stars tells me that the...
  12. Zack K

    Understanding Starlight: White Light, Double Slit Experiment, and Color Emission

    I've been recently watching videos about white light and the double slit experiment and how it creates a rainbow. It let me to some confusion and I would like some clarification. My questions are: Do all stars like ours produce white light? Would red giants only emit red light and blue giants...
  13. A

    I Why does temperature always rise in stars?

    Hello everyone, I know that pre-main sequence stars do heat up because of gravitational contraction, and the increase in internal energy (and so in temperature) comes from this shrinking and is governed by the virial theorem (...
  14. Oganesson

    I Can someone please explain to me Leavitt's law?

    And how we use it to calculate distances from stars to the earth.Thank you
  15. AshUchiha

    B Why do stars experience gravity more than its mass?

    I will try to be as literal as I can. I was researching about the black hole, according to NASA and much other organisation a black hole form when a star collapses under its own gravity. "Stellar black holes form when the center of a very massive star collapses in upon itself". Gravity is...
  16. K

    I Fixed Stars Moving Faster Than Light? Problem?

    If I spin around in an open field at night and look up to the stars they appear to be moving relative to me. Additionally, they are very far away and trace out a giant arc length in a very short time (S=rθ). With respect to me, these stars are moving faster than light. Is this a problem? Has...
  17. SF Alba

    B What is the maximum lifespan of a B-class blue giant star?

    Hi. I couldn't find good information of this online, so I'll ask here. I'm wondering what the maximum possible lifespan of a particular star. Here's what is known of it: It's a B class-blue giant star. Its surface temperature is 25,000 K. (This is very vague and may offer no help but:) It has no...
  18. C

    Do Photons from one star affect other stars?

    I am not a physicist but have heard that photons from the sun effect the flight path of asteroids? I was just wondering if these same photons would ever be able to effect nearby star systems? Therefore could there be a slight push between star-systems pushing them further apart? (Does this come...
  19. J

    Nuclear Fusion in Stars & Element Formation

    Homework Statement Ok, these questions are very simple but they are really bugging me and I would greatly appreciate an explanation. Question 1 is "which of the following elements must have been made in stars". The options are hydrogen, helium, carbon, oxygen and iron. Question 2 is "which of...
  20. H

    B Did stars come before all life?

    If so, what is the evidence? Sources are appreciated.
  21. Battlemage!

    B How important is gravitational red/blueshift for stars?

    I know that the speed/acceleration of a star can be measured using redshift/blueshift caused by the motion of stars, but I just learned that gravitation also causes redshift and blueshift. Would this not cause some uncertainty into the determining how fast a star is moving away from us? If so...
  22. Seanra

    I Could "reverse entropy stars" exist in our universe?

    My lecturer claimed that "reverse entropy stars" could exist in our universe. One of the examples he gave was that if you exposed some sort of detector in the direction of a hypothesized reverse entropy star, you could determine if it existed by whether it "sucked" photons out of the detector...
  23. Barely_Conscious

    I How would one know whether a star would be observable?

    I'd like to answer this yes or no question for a number of objects: "Is this star, at any point between these two times, going to be above the local horizon?". Say, I'm at the prime meridian at a latitude of 50 degrees, and I want to know whether, between the sidereal times of 11:00:00 and...
  24. W

    I Why there are different size stars?

    I just read something about the theory of stars formation that left me puzzled. I am easily puzzled so please bear with me. The theory goes somewhat like this: There is s nebula (gas) that is contracting and increasing mass and pressure at its center until they are so big that a nuclear reaction...
  25. J

    Initial Mass Function Question (white dwarfs, neutron stars and black holes)

    Homework Statement Calculate the number of white dwarf, neutron star and black holes the galaxy will have after 5Gyr of evolution. M<8(solar mass) for white dwarf 8(solar mass) < M < 40(solar mass) for neutron star M > 40(solar mass) for black hole Initial mass function is ξ(M) = AM-s...
  26. Stephanus

    B Can we see individual stars in the Andromeda Galaxy?

    Dear PF Forum, Just out of curiosity :smile: Can we (through telescope or HST for example) see stars in Andromeda Galaxy? Is the Andromeda Galaxy the closes galaxy to us. Can we really be sure that there is no other galaxy across Milky Way because our line of sight is blocked by clusters of...
  27. Astronuc

    News Panama Papers - Huge tax leak exposes Putin aides, world leaders, stars

    Panama Papers Some highlights so far, I wonder if any US politicians or businessmen are in the records.http://www.usatoday.com/story/money/2016/04/03/german-paper-massive-financial-leak-reveals-offshort-accounts/82586798/ The Panama Papers (11.5 million documents, 2.6 terabytes)...
  28. T

    B Binary Stars Observable with Binoculars: Periodicity

    Are there any binary stars that can be seen with binoculars which have orbital periods you can observe from week to week or month to month? tex
  29. Janus

    Insights How Representative is the Night Sky? - Comments

    Janus submitted a new PF Insights post How Representative is the Night Sky? Continue reading the Original PF Insights Post.
  30. Allen_Wolf

    B Gas Giants & Stars: Do They Have a Solid Part?

    Are the gas giants and stars only composed of gas particles? Don't they have a solid crust or something? If they don't have a solid part then how can they exist alone?
  31. Alltimegreat1

    Direction of stars' orbits around galactic center

    Do all stars in the Milky Way orbit the galactic center in approximately the same direction? Also, toward which Zodiac constellation is the Sun moving in its orbit?
  32. Elbert Anstein

    B Pulsars and Neutron Stars debunked?

    Now kindly pardon my ignorance but I hope one of you can explain to me how this can be possible. A rotating neutron star or pulsar have been observed to spin at between 10 to 700 times per second. Now before I proceed further I like to draw your memories back to the merry go round we played as...
  33. Z

    Stargazing Movement of Stars seen from the North Pole

    From the perspective of someone at or near the exact north pole (where, for all practical purposes, they are not rotating), ignoring the gradual change of the stars as the Earth orbits the sun, would the stars appear to move at all in the night sky? Or would they be stationary because the...
  34. J

    I How to Convert Plummer Distribution of Stars Parameters into Standard Units?

    Hi all, I refer to the following pdf document, in particular the appendix: http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1974A%26A...37..183A&link_type=ARTICLE&db_key=AST&high= End goal is to distribute N stars each having mass m (looks like equal mass stars is the easiest scenario)...
  35. wolf1728

    I Are almost all stars in the night sky brighter than the Sun?

    I was thinking about adding another page to my website concerning the nearest stars, the brightest stars, etc. In a list of the nearest stars, the vast majority are brighter than the Sun. (I looked for stars with an absolute magnitude that was greater than the Sun's (4.85) and had a visual...
  36. henrco

    Rocket in space travels between two stars

    Hi, Could I please get some guidance on if my approach and solution are correct here. I feel I'm on the right track. But even though the answer to part b) feels right, it would be helpful to get advice. Homework Statement A space rocket travels between two stars separated by 10 light years...
  37. J

    B Orbital parameters of stars orbiting Sagittarius A*

    I was going to try and do an animation of stars orbiting Sagittarius A* but can't seem to find any useful data for it. The Wikipedia page has some data https://en.m.wikipedia.org/wiki/Sagittarius_A* and was trying to reconcile this with https://en.m.wikipedia.org/wiki/Orbital_elements since I...
  38. T

    B Intergalactic Stars: Are There Single Stars Between Galaxies?

    Are all stars contained within a galaxy? Or are there some single stars between galaxies? If there are intergalactic stars how were they created since I thought galaxies provided the creation mechanism? tex
  39. W

    What happens to energy of the stars?

    Several days ago I started thinking about the mystery of dark matter (yup, it was after LIGO discovery, and we all do it from time to time, right?). Then I came up with a strange conclusion that within our galaxy, there should be an equivalent of about 24 000 stars that we just cannot see--in...
  40. E

    Stars we see are actually their past?

    One questions that I am really confused with! If the sun is 8 light minutes away, this means that whenever we see the Sun, we are actually looking at how the Sun looked like 8 minutes ago if I am not wrong. If we look at a star that is let's say 15 billion light years away from the earth, is it...
  41. T

    Formation of stars and non-conservation of angular momentum

    Good morning all, Recently in a modern physics course of mine, my professor was covering the topic of energy levels and ionization energies and it included a diagram very similar to this one: While it is interesting to learn that these diagrams correspond to a very specific and strict set...
  42. JoAstro

    Two stars with the same mass equals the same luminosity?

    As the Stephan-Boltzmann's Law says, the minimal change in a star's mass would have a massive effect on its luminosity, but can necessarily two stars with the same mass have the same luminosity?
  43. B

    B "Gravitational Compression in Neutron Stars"

    What happens to the neutrons in a neutron star as it collapses Into a black hole?
  44. T

    What would happen if two stars collided?

    What'd happen if two stars collided? Could it happen? Would the speed/mass change the result?
  45. Jonathan Scott

    X-ray bursts might not happen for larger neutron stars?

    A method of definitely distinguishing a neutron star from a possible stellar black hole is that it produces X-ray bursts, which have a sharp rise time and may last for an extended period. I had previously thought these occurred when hydrogen fell to the surface and was immediately fused to...
  46. Stephanus

    Removing Stars from Google: Tips & Tricks

    Dear PF Forum, Anybody knows how to remove the star? I forgot when I unintentionally put that star, now I want to remove it. But I can't. Some websites suggest by inserting some HTML (or Java script? Can't tell the difference) script. Is there an easy way withouth signing off google? Thanks...
  47. Teichii492

    Stars in the early universe and stellar processes

    Hey PF, Since there are stars that can be powered predominantly (>50%) by the CNO cycle, which requires carbon as a catalyst, and i understand the core temperatures of these stars is about 106 K. Does this mean that stars where the triple-alpha process is dominant (108 K) had to exist and die...
  48. NihalRi

    Spectra of Stars: Reliable Data & Quantitative Analysis

    I'm looking for reliable data of the spectra of different stars, anyone know any good resouces? Quantitative data would be best because I want to analyze it myself (not just find out something that was already on the site). Thanks in advance :)
  49. A

    Neutron Stars from White Dwarf Mergers: Under 1.4 SM?

    I think this could be a good chance that we see how neutron stars form from White Dwarf mergers. The combined mass of the two are less that than required for a type Ia Supernovae. Won't they merge and collapse into a neutron star? What does everyone else think might happen. I want to be sure if...
  50. Astrodent

    How many stars we can see from Earth?

    Greetings, Hi, I am new member of Physics Forums. Here is my question: How many stars can we see from Earth? and also If you know, How far can we see? Thanks!
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