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. takacszoltan87

    I Expanding Universe without dark energy

    Dear all! I'm not a scientist, but I'm thinking about something, I'm curious, is it possible to believe that our universe is in a vacuum, the stars are moving away a not because of the dark energy but becouse it behave like a luffbaloon in a vacoom. Sorry for noob questions.
  2. Vanadium 50

    I Where Are the Missing Black Holes in the Milky Way?

    Where should the nearest black hole be? Something like 0.1% of stars end up as BH's, so that suggests about 100 million in the Milky Way. The easiest thing to do, instead of a complicated geometry problem, is to recognize that the cube root of 0.1% is 0.1, so that we expect BH's to have...
  3. M

    Two stars orbit their common center of mass

    Why do we add the two masses (3M+M=4M) and use that for M in the equation of kepler's 3rd law? Namely why is it T^2=4pi^2R^3/G(3M+M)
  4. K

    I How does gravitational self-interaction affect the Milky Way Galaxy?

    It is believed that gravity interacts with itself. I assume that gravity between stars increases. Does gravitational self-interaction change the galaxy's shape or increase the rotation curves of stars?
  5. Hak

    I Nuclear reactions in the Sun and other topics on stars

    I don't know if the Forum in which I am posting is right, if it is not please freely move my question to the Forum you think is most appropriate. You argue that nuclear reactions occur in the Sun, which produce the energy that spreads out into space in the form of light. A friend of yours...
  6. jeffinbath

    I Surely the 2nd Law of Thermodynamics is defied by gravity?

    As our sun and the other stars evolved from gravitationally led aggregations of hydrogen gas which permeated our early universe then that is an example of a high entropy system becoming a low entropy system and the so-called "arrow of time etc." was reversed?
  7. ZX.Liang

    I What are the conditions under which stars can radiate coherent light?

    Some papers mention the coherent radiation of stars, such as this one: https://iopscience.iop.org/article/10.1086/519790/pdf I want to know under what conditions can a star exhibit coherent radiation?
  8. Asem

    Angular Velocity of a Large Pendulum on Earth as seen from the stars

    I don't understand the question. how am I supposed to find the magnitudes and directions of the velocity from the figure?
  9. Ken G

    A As stars contract, why does total entropy rise?

    The FAQ by @bcrowell cites an explanation by physics netizen John Baez as to how entropy rises when a star loses heat and contracts. However, the linked explanation falls short of describing the key role that gravity must be playing. The FAQ by @bcrowell discusses why a low-entropy state of...
  10. Astronuc

    B Vulpecula, pulsars and neutron stars

    I was listening to a Star Date podcast regarding the constellation Vulpecula and learned about the discovery of pulsars and neutron stars. https://stardate.org/radio/program/2023-03-19 The first neutron star was discovered in Vulpecula in 1967. https://en.wikipedia.org/wiki/PSR_B1919+21 In...
  11. S

    I What Milky Way stars could go supernova to < +3 at any time?

    (I use +3 as the cutoff as that seems to be the limit of easily visible stars.) Yes, I know that Betelgeuse could go up in a < -10 blaze of glory, but I wonder what other ones are out there. On a side note, how fast could the big observatories move to it to observe it? And how quickly would...
  12. F

    B Forming Stars/ Brown Dwarfs by photo-erosion

    Hello, do anybody knows something about the formation of Stars and brown Dwarfs due to photo-erosion? If prestellar cores form in a molekular cloud with some O- and B-stars, the gas or the hydrogen gets ionised and this somehow stops the protostar from akkreting more mass. Why does that stop...
  13. L

    B Are the stars of the constellations stationary?

    Why is it that I can still use my 10 year old planisphere, if the stars are in motion?
  14. FactChecker

    B Are the stars that we see in constellations within the Milky Way galaxy?

    Are the stars that we see in constellations within the Milky Way galaxy? Should I assume that they are close and within the Milky Way?
  15. S

    I How far away are stars in a typical star cluster from each other?

    I'm trying to grok what this would look like to an observer on an Earth-like planet around a star in the center of such a cluster.
  16. G

    B Projected linear separation between companion stars

    A quasar with a bolometric flux of approximately 10−12 erg s−1 cm−2 is observed at a redshift of 1.5, i.e. its comoving radial distance is about 4.4 Gpc. Assume that the quasar in the previous question is observed to have a companion galaxy which is 5 arcseconds apart. What is the projected...
  17. T

    B Who was the first person to recognize the stars as very distant suns?

    The first person to realize the night sky is filled with stars similar to our sun was a great leap in imagination. Cassini in 1672 measured the sun distance by parallax measurement of Mars so I assume he knew. But you didn't have to know the solar distance to speculate that the stars were more...
  18. Barbequeman

    Surface density of stars in a Galaxy

    a.) The scale length of the disk is the length over which the surface density of stars decreases by a factor of e. In this case, the surface density decreases by a factor of 10 over a distance of 9 kpc, so the scale length is 9 kpc. The surface density of stars at a radius of r from the center...
  19. AotrsCommander

    Help requested on long-term habitable-zone stars

    I am currently writing another entry for a massive lore project (you may have seen the previous thread on the shape of the galaxy). I am usually pretty good about getting the details accurate (or at least explaining around them), but my sources I am working from to write up are from about...
  20. Sciencemaster

    I Database of binary star data info within 10 PC of Earth

    I'm looking for a database of binary stars within 10 PC of Earth, including information such as eccentricity of orbits, their distance from one another, etc. I'm hoping to find a list with this information, or just a collection of pages with this information. I've tried Simbad but I can't find...
  21. I

    B Distance of Stars / Size of the Universe calculation

    Hi, I am a new user, This question is bothering me for a long time and now with all the Webb telescope hype I need to ask: It sounds very logical to think that a star that is a billion light years away is seen as it was a billion years ago because the light took 1 billion years to get here...
  22. Rikudo

    Fictitious force in a binary stars

    I have a difficulty in understanding the question. Fictitious force is a force whose motion is described using a non-inertial frame of reference. Which frame is the question referring to?
  23. H

    B Amazing Rings: Intersecting Stars Create Spectacular Gas Rings

    Two stars with orbits that almost intersect create concentric rings of gas.
  24. BWV

    B Questions about Supermassive stars in the Milky Way Galaxy

    Looking at the list on Wikipedia and reading about the theoretical limits of ~150 M for population 1 stars, are the stars in the Wikipedia list above that all population 2? the large stars on the list are either in the LMC or in a region of the Milky Way called Westerhout 49 So the LMC given...
  25. Cerenkov

    B Where can I find information about Population III stars?

    Hello. I've performed a Google search on the subject of Population III stars and come up with some results. But I'd now like to go a little further. Could I please be directed to articles and/or papers on this subject, so that I can further my interest? Thank you for any help given. Cerenkov.
  26. Cerenkov

    B Would like to explore the Henry Draper catalogue of stars

    Hello. I would like to explore the Henry Draper (HD) catalogue of stars so that I can improve my understanding of the host stars of many exoplanets. When I went to Wikipedia, they provided a link to this site. https://vizier.cds.unistra.fr/ The VizieR, TAP and XMatch icons appear to lead to...
  27. PainterGuy

    B Different stars visible from northern and southern hemispheres

    It's a general question and not even sure if I should be posting it over here in section. Anyway, one cannot see Big Dipper from some countries in Southern Hemisphere, countries such as New Zealand and southern parts of Australia. People in Northern Hemisphere and Southern Hemisphere get to see...
  28. Astronuc

    B NGC 4590, more than 1,400 stars found in various evolutionary stages

    Ultraviolet bright sources inspected in NGC 4590 https://phys.org/news/2022-02-ultraviolet-bright-sources-ngc-stars.html Using India's AstroSat spacecraft , astronomers have inspected ultraviolet (UV) bright sources in a globular cluster known as NGC 4590, which had previously been viewed with...
  29. Arman777

    Stargazing 3D/2D Universe Model (DSO, stars etc.)

    Hey all, I am looking forward to knowing some software/sites that show an extensive collection of the DSOs. I want it to be 3D, but it can also be 2D. I know/tried some of these, Stellarium - 2D Celestia - 3D https://www.legacysurvey.org/viewer#IC 2095 - 2D Gaia Sky (I have tried to download...
  30. J

    Cesium-133, the Sun, Moon, stars, or.... T.P. Scott-1000?

    Early in the pandemic there was, at least in the U.S., a concern about toilet paper shortages. It all seems so quaint now but there were actually runs on t.p. in supermarkets and people were hoarding it. There are of course other ways to clean up afterwards. I reckon that most people in the...
  31. A

    B Mass transfer between neutron stars in a binary pair

    When we read about the mass transfer between neutron star pairs in a binary system, how is it that the one receiving the matter can increase its spin rate. Adding mass to a spinning object ought to slow due to conservation of momentum. Where does the energy come from?
  32. NnnTech

    Why does it look dark between the distance stars at night?

    Hello , I have to do some ''homework'' on the nature of light ! I have to write everything I can think of ! When I look in the night sky , between the distant stars it looks observably dark when there is electromagnetic radiation filling that space . Why does it look dark ?
  33. DaveC426913

    B Stars massive enough to evolve into black holes

    I keep seeing this figure of 3-4 Solar masses is all that's required for a star to end its life by collapsing into a black hole. Since the sun is a pretty average star, that would suggest that most of the stars in the universe (like >50%) will end their lives as a black hole. The implication...
  34. elcaro

    B Can we measure acceleration of galaxies and stars?

    As for example we see a large void, the Great Repeller, which in fact is an underdense region, and with respect to this region, matter seems to be repelled by this region. The explenation for that is that matter outside that regions pulls on the matter inside it. But if that is really the...
  35. N

    B EM radiation creation within stars

    Having had a look at the following video of the dipole antenna and the creation of EM radiation, which I completely understand, I had a look at the link http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/procyc.html#c1 and the hydrogen fusion process within a star. Looking at the process how do...
  36. Cerenkov

    B What kind of orange K dwarf stars are prone to flaring?

    Hello. I would like to discover if there is any kind of relationship between the 'lateness' of K-type dwarf stars and the degree to which they flare. By 'lateness' I am referring to the classification of spectral types. So, K0 to K4 would be considered to be 'early' and K5 to K9 would be...
  37. Antman0115

    B Black Holes & Stars: Quantum Entanglement Possibility?

    Would it possible that black holes share a quantum entanglement with stars(such as white dwarfs), and the reason we observe the tunnel closing is the star dies out/explodes? I understand that there are different types of black holes and stars, varying in size and properties of mass/spacial...
  38. P

    I Why are emission spectra of stars rarely shown?

    According to this link you just have to anlayse the light that isn't coming from a place on the star that has a light the source directly behind it e.g wouldn't looking at light from the outer edge of star give you an emission spectrum? http://www.thestargarden.co.uk/Spectral-lines.html
  39. C

    Uniform Density Stars: Struggling with Understanding

    I don't know where to start. This chapter is incredibly confusing for me.
  40. K

    I Will stars on the other side of the galaxy affect gravity here?

    Does intervening mass between two stars decrease the gravitational attraction between these two stars? Is gravity a local phenomenon in that local mass interacts with the surrounding gravitational field caused by the local star and distant stars?
  41. A

    I How do Limb-Darkening curves differ at two different wavelengths?

    Does the limb-darkening curve fall off faster at shorter wavelengths or at longer wavelengths?
  42. K

    B "Snow" on comet 67P/Churyumov-Gerasimenko is actually stars

    You may have seen: 10 Space Pictures That Look So Good You Won’t Believe They’re Real Starts With A Bang Ethan Siegel 9.) Snowy weather on comets. The ESA’s Rosetta mission witnessed cometary “snow” firsthand. However, landru79 (twitter) has rearranged the frames to show that most of the...
  43. Cerenkov

    B Tidal locking for planets of K type (orange) dwarf stars

    Hello. If its possible I'd like to find out more about tidal locking for planets orbiting K type dwarf stars. Specifically, at what distances from their host stars would exoplanets have to be to avoid becoming tidally locked. I'm specifying K dwarves because, from what I've read (see...
  44. P

    B Interested in Black Holes, Neutron Stars, and White Dwarf Stars

    How did you find PF?: duckduckGo search. My math is very weak and I don't like explanations done using math. I read books with very little math. I try to use reason based on what I've read. My understanding is time slows down in gravity and it will actually stop at the event horizon (see...
  45. K

    I Do stars in the ecliptic seasonally change brightness?

    Has any astronomer observed any seasonal changes in the brightness of stars lying close to the ecliptic plane? (Other stars are: d Cnc b=+0.08, mag.=+3.9; d Gem b=-0.18, mag.=+3.5; a Lib b=+0.33, mag.=+2.7.) Once a year these stars are occulted by the Sun. What happens from the date of...
  46. bbbl67

    I Strange quarks, Strange stars and Strangelets?

    So various articles and videos suggest that if a piece of strange matter, or a strangelet, were to touch the Earth, the entire Earth would eventually get converted into strange matter too. Now, from what I've read strange quarks have a half-life of ##10^{-10}## s, so I can't see how it would...
  47. N

    Stargazing What do stars actually look like from up close?

    I have been wondering what certain stars would look like from a closish distance. The kinda size with about the same angular diameter that the Sun has from Earth or bigger. I was mostly wondering about red giants, red supergiants, and red dwarfs. Like… Would they look red from that sort of...
  48. N

    I Blackbody colour of metals versus stars

    Hello. I am new to this forum and joined because I am at home nerding out trying to work something out. Why do white hot metals seem to be much cooler than white hot stars. The attached picture is from Wikipedia relating temperature of a hot metal to its temperature. For example a red giant...
  49. T

    B Where do the electrons go? (in stellar nuclear fusion)

    I have to give a presentation about natural Radiation and I am very happy about it because it includes Astrophysics. I want to explain to my audience how the stars produce cosmic rays. I thought about explaining to them how nuclear fusion and that kind of stuff works but then I realized that I...
  50. Buzz Bloom

    I Q: Volume of the largest ellipsoid in space which contains no stars?

    The answer to the primary question in the summary is the first step in seeking an answer to a more complicated question I plan to post in a separate thread later. This more complicated question is a consequence of the thread...