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. O

    I What did we know about stars in 1950?

    I was reading an old Asimov novel called Pebble in the Sky, and it somewhat irked me that Sirius and Aldebaran were described as major stat systems of the Galactic Empire. Sirius is too young to have had time to develop planets with higher life (if evolution on Earth is anything to go by), and...
  2. DennisN

    Stargazing Can a Mobile Phone Capture the Beauty of the Night Sky?

    Behold my first photos of stars...:smile: Thanks to PF members and PF threads, and reading about astrophotography, I succeeded the first time. This was just some tests I did with my mobile phone, LG G4, which has a pretty decent camera which can be used in manual mode, i.e. I can set the ISO...
  3. S

    I Do stars have any lower mass limit?

    Just read an article about a discovery of the smallest/least massive star in the Milky Way galaxy. The star has 85 times the mass of Jupiter and is known as EBLM J0555-57Ab located about 600 light-years from Earth. The entire article here -...
  4. Strange design

    I Observing Orbital Speed of stars within Galaxies

    Can someone point me to a resource that describes, or describe, how the orbital speed of stars within galaxies are observed and measured? I'm struggling with how these values can be attained with any confidence in their accuracy. Thanks :)
  5. V

    B Inclinations of star orbits in the Milky Way

    Do all stars orbit around the black hole in the same plane? Or some stars can have the same orbital radius but slightly different orbital inclinations?
  6. TheQuestionGuy14

    B Question About The Stars -- Can they have died before we see their light?

    Stars have always interested me, I view them every night with my telescope. One thing really interested me though, if star's light can take thousands of years to reach our planet. I was wondering, does this mean that the light we see could be the light of a star that has died, maybe for hundreds...
  7. A

    Stargazing Where can I find high resolution photos of distant stars?

    Hello I've been digging around for high resolution photos of distant stars now for awhile. The search has been pretty difficult. If what I've been reading is correct then there appears to be a limit to viewing small objects like stars, if this is true can someone help me understand the reason...
  8. Chatterton

    Figuring out which stars an exoplanet's poles point to

    I know Earth's north pole points to Polaris and during the 26,000-year precession cycle, it will also point to Demeb, Vega, and Alpha Draconis. But... Is there a reference that'll help me figure out what Proxima Centauri b's most likely pole star candidates would be? I know, so many variables...
  9. S

    I Where to find full core-to-surface profiles of stars?

    I need to run some computations using an initial star. I could evolve one using MESA but it'd be very convenient if I could download a ready made profile/model of a star which details things like mass, dens, temp, etc, from core to surface. I've googled for a long time but I just keep getting...
  10. Chrono_13

    B A Question about tiny tiny stars, and radiating heat

    A brilliant person once said “First get your facts strait, then proceed to distort them at will” or something like that. My question, is what would happen if say… a sun of relative density to our own, but only the size of a golf ball, just appeared in say… downtown LA, and sustained at that...
  11. U

    B Cepheid Variable Stars and Space-Time Warping

    After going through the starting parts of Astrophysics, (and excuse me if I completely mess this up), that if the apparent brightness of the star decreases due to the loss in energy and contracts then expands again over a certain period. Then as the energy released has a mass equivalence, then...
  12. J

    I Understanding the Formation of Stars: A Look at Gravity, Gas Pressure, and Heat

    Specifically how did the stars collapse into themselves. If gravity varies by the distance squared and gas pressure varies by the volume cubed, how could a cloud of hydrogen, or any gas for that matter, collapse into itself? Not to mention the fact that the heat increases which would also...
  13. R

    I Are quark stars a realistic solution to the black hole singularity problem?

    Is this a realistic solution to the black hole singularity problem, Going beyond that, neutrino stars?
  14. Guilherme Franco

    I Where to find in-depth info about each star's composition?

    Hello. I couldn't sum my entire question in the title, so here it goes: I have a friend that needs information about the content of Iodine and Molybdenum in stars (from as much stars as he can). There are references to the content of many stars in books and papers, but most of them don't give...
  15. J

    B Are Black Holes Actually Giant Neutron Stars Cloaked in an Event Horizon?

    I was just wondering if there is anything to suggest that black holes are anything but giant neutron stars cloaked in an event horizon created by their own gravity. I mean if a neutron star is just on the cusp of having enough mass to be a black hole, and then gains that mass, what's to say it...
  16. T

    I Math doesn't add up: stars + planets < particles in universe?

    I see they finally counted all the particles in the universe, it's10 to power of 80 They also counted all the planets: 10 to power of 24 and also counted all the stars: 10 to power of 24 and also counted all the atoms in the Earth: 1.3 x 10 power of 50 Good work everyone! Now, I'm trying my...
  17. D

    Stargazing Apparent vs actual position of stars

    When we ‘see’ the Sun, from here on Earth, what we are actually seeing is the Sun as it was over 8 minutes ago when the light we currently observe first embarked on its journey towards us. In that time the Sun has in fact arrived at a position about two degrees east of the place it now appears...
  18. N

    B Do neutron stars have a minimum volume?

    Do neutron stars have a minimum volume? Anything "in the way" of perhaps baseball sized neutron star? Or would something like that be an impossibility? How neat to have one in a laboratory...or not lol
  19. Colm

    B How do we know the stars are not our own light falling back.

    How do we know that the stars we see, or some of them are not the light from our own system falling back, and that we may be looking at light from our own star/solar system/ galaxy that has taken millions of years to fall back on its point of origin?
  20. Jamison Lahman

    I Are White Dwarfs Stars? Understanding the Classification and Characteristics

    Simple enough question, are white dwarfs stars? I know white dwarfs are the remnants of former stars, but is it correct to classify them as a current star? My understanding was that nuclear fusion was a deciding factor into whether or not an object was a 'star' and white dwarfs were void of any...
  21. J

    I Questions about the lifecycle of stars

    Hello, I have been following lectures found here on the end of the lifecycle of stars (while trying my best not to get distracted by his bow tie) and I have some questions on this. Some of them being general questions and some a bit more detailed. 1. First a more general quesion: I’ve read...
  22. A

    Planetary tidal locking of Close Binary Red Dwarf Stars

    Hi, I'm new to the forum and have an idea for a novel. I can't find anything on this question so I thought I'd sign up for a forum to ask. I'm not a scientist and lack much of the higher technical knowledge. Here's my question: Most planets in close orbits (they'd have to be to support liquid...
  23. J

    I Mass and temperature relation in stars

    Hoping someone can help me here, I'm only a student so I'm sorry if my question is badly worded. I'm doing my maths dissertation on a binary eclipsing star and I'm trying to work out the mass of one of my stars. I know the B-V value and effective temperature, and I believe the equation I need...
  24. C

    How can neutron stars have magnetospheres?

    Neutron stars are neutral by definition. How can they have magnetospheres which are polar by definition?
  25. P

    B Dark Matter Stars: Burning Hot and Invisible

    ok so this has been on my mind for a while. If the hotter a stars burn or flame the further it goes on the spectrum correct so what if it burned so hot it is literally invisible wouldent that be considered a dark matter then it's a dark matter sun
  26. D

    B Generating Plausible Star Systems

    I'm trying to write some code to generate plausible star systems, but so far, I only have a main sequence star generator, and even with that I don't know how good the mass-radius relationships are. So, I'm coming here to ask for some formulas or pointers as to where I can go to find the math to...
  27. G

    B Cepheid variable stars cause of light variance

    Have been reading about cepheid. Can I just check I've got this? 'When at their hottest (when gravity has squashed them) have more doubly ionised He so star becomes more opaque' Now was just wondering what the main mechanism is here? Because He2+ doesn't have any electrons left to absorb photons...
  28. PhysKid45

    B Exploring Blue Light: The Mystery of a Low-Wavelength Star

    For my astronomy class we were asked a question about a star that is 50,000K and it's peak wavelength. This was easy enough to find using Wein's law. The answer, however, is well below a human's threshold for visible light and we theoretically would not be able to see the star, but apparently it...
  29. RyderP

    Super-Earths around binary dwarf stars

    I may be writing a story, set on a rocky planet, 15 times the size of Earth, around a binary dwarf star (one red, one orange). What kind of flora and fauna can I expect?
  30. Brunolem33

    B Formation of Stars: Questions on Range & Equilibrium

    Recently watching a documentary about stars (how the universe works), I was wondering about two things. The first one is: how is it possible to have stars with such a wide range of sizes? As far as I understand, stars are created in swirling gas clouds, nebulae. When the accumulation of gas...
  31. L

    Stars in the same constellation

    Do any of the constellations have two or more stars in them that happen to actually be close to each other (10 light years or less)? This excludes binary star systems of course.
  32. tarkin

    Solving Binary Star Problems: Tips & Strategies

    Homework Statement http://prntscr.com/dsd7ea Image attached Homework Equations For circular orbit, r = Pv / 2 pi , Where P = orbital period and v=orbital velocity r' = r sini , where i is unknown angle to plane of sky.The Attempt at a Solution [/B] I'm really not getting these binary...
  33. A

    B Mini Neutron Stars: What is the Smallest Possible?

    I know there is theoretical possibility of making mini black holes in particle acclerators, but what about mini neutron stars, or even mini quark stars? I realize the neutron decays in 15 mins if left all alone. What is the minimum number of neutrons needed inside a nuclei for it to be stable...
  34. virgil1612

    I Nebulosity around Pleiades stars

    Hello, I'm curious about the origin of the nebulosity that can be seen around the main stars in the open cluster Pleiades (M45). Is it a residual of the huge cloud from which these stars were born not so long ago (astronomically), or it just happens that these stars pass through an interstellar...
  35. Tom MS

    B The Semi-Major Axis of Binary Stars

    Wikipedia seems to think that a binary system is defined by a single semi-major axis, but I've seen other sources such as hyperphysics that define it using two semi-major axes. Is the semi-major axis of the system simply the average of the two?
  36. A

    I Percent of stars in our galaxy that belong to clusters?

    Of all the stars in our galaxy, how much of them belong to clusters?? Is our sun rare in terms of not belonging to any cluster? I can't find any sources on that
  37. Hena Adlakha

    Can we use technology to view the past through stars?

    Hi:smile:. I am new here! My first question: We are watching those stars which were present many years ago in the sky. So, basically, we are viewing the past. Now, my question is that, if we develop some kind of technology through which we are on such a 'star' and are magnifying earth(just...
  38. D

    Proper Motion of Stars: Solving Daniel's Calculation Problem

    Hi I have a question about "proper motion of stars". At this page (http://www.astronexus.com/a-a/motions-long-term) there are a lot of examples how you can measuring the proper motion of stars. But at "Point 4 - Turn these velocities into Cartesian velocities" i have some different results...
  39. dykuma

    Why Are Stars Hot? Exploring the Heating Process of Stars

    Homework Statement Why are stars hot? Homework Equations Non specifically, it's a conceptual question. The Attempt at a Solution I have always heard that stars are hot because (as a quick very basic example) stars are large collections of mass which collapse in on itself. Eventually the...
  40. T

    Distance Between Two F0 Stars: How to Calculate?

    Homework Statement A distant F0 star is 11.3 times less bright than a nearer F0 star that has a stellar parallax of 0.05 arcseconds. What is the distance in parsec of the more distant F0 star? Homework Equations $$d=\frac{1}{D} {pc}$$ $$B_0/B_1 = 10^{\frac{(m_1-m_0)}{2.5}}$$ The Attempt at a...
  41. L

    B What is the Hoyle State of Carbon and How Does it Contribute to Star Fusion?

    A popular video I just watched described Fred Hoyle's discovery that the elements of the universe are created in stars. Key to his theorizing was the prediction that fusion would produce of a new state of carbon that had never been observed and which theory predicted would be unstable. Hoyle...
  42. L

    B Neutron Stars and Angular Momentum

    Some observed neutron stars rotate hundreds of times per second. Speeds at the surface of these stars are as much as 15% the speed of light. These huge speeds are generated because angular momentum is conserved when a large rotating pre-super nova star collapses into a neutron star. The...
  43. |Glitch|

    I Are Red Dwarfs Home to Earth-Sized Water Worlds?

    It has always been my impression than spectral type M stars were notorious for being flare stars. As a result of their small radius and relatively low effective surface temperature, the Habitable Zone has to be relatively close to the surface of the star and small in size. As a result, it...
  44. Jim Hasty

    A Redshift data on stars the same distance from earth

    Can anyone in the cosmology community direct me to hard research data that specifically demonstrates: there is no detectable redshift difference in stars that are the same distance from the Earth but in all different directions. This is of course related to 'does the universe have a preferred...
  45. Jamison Lahman

    Core Mass-Luminosity Relationship in RGB Stars

    Homework Statement Low-mass stars like the Sun obey the core mass-luminosity relationship as they burn H in a shell and climb the RBG (Red Giant Branch). What is the energy released per unit mass when fusing hydrogen into helium? Homework Equations The core mass-luminosity relationship: $$...
  46. newjerseyrunner

    B Do Stars Precess? Exploring the Planet 9 Claim

    I read that part of the planet 9 claim is that the planets orbit 6 degrees off from the rotation of the sun. Since we're we're made from the same disk, it should be the same. Isn't a simpler explanation for at least that claim that the sun itself simply turned? Don't all spinning object go...
  47. Clara Chung

    B Line absorption spectrum and stars

    Why can't hydrogen gas on the stars be detected by using line emission spectrum of hydrogen. Why must we use line absorption spectrum to detect?
  48. just wandering

    I Why don't any stars "look" red? red shift

    Universe is expanding/galaxies moving away from each other and far away galaxies are moving away from us faster. so the light is redshifted. .."at very large redshifts, much of the ultraviolet and visible light from distant sources is shifted into the infrared part of the spectrum. This means...
  49. V

    B Entropy change during the formation and death of stars

    The stars formation decreases the entropy but the radiations given out increases the overall entropy i am said ...but once the fusion processes are over the star no longer gives out radiations...and still the volume in which the matter particles of the star can be becomes less as the star...
  50. T

    I Why are stars grouped in galaxies?

    I have been thinking at this for a couple of days now: why are stars grouped in the massive collections that we call galaxies? I can assume that in the very early Universe, matter was grouped in these areas, that matter interacted thus resulting in the formation of stars. Then, the...
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