What is Star: Definition and 1000 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. samjohnny

    Calculating distance to a star using magnitudes

    Homework Statement Homework Equations ##m = m_0 − 2.5 log_{10} f## where ##m## is the absolute magnitude of an object, and ##m_0## is the zero point. The Attempt at a Solution I'm having difficulty with is part b, the answer to which I believe is required for the subsequent parts. I...
  2. A

    I What is the average number of planets per star?

    What is the occurrence rate of planets? I'm not talking about earth-like planets, but any planet in general. I can't find a source that examines this number, all I can find is the average number of earth-like planets. Can anyone lead me to a useful paper/source? Thanks in advance
  3. jtbell

    50th anniversary of the original Star Trek

    On September 8, 1966 the first episode of the original Star Trek series was broadcast. I saw it when I was in junior high school (middle school). Tonight I'll pull out my DVD set and watch it again. It's been several years since I last did it. Maybe I'll make it a regular Thursday thing and do...
  4. N

    B Emission and Absorption Spectra

    When you heat things up, they emit specific wavelengths of light, right? Like when you heat up sodium, it emits yellow. But don't things emit shorter wavelengths of light at higher temperatures? Like how hotter stars are blue and colder stars are yellow. Since stars are mostly hydrogen...
  5. Garth

    B 'Strong signal' from sun-like star sparks alien speculation

    Hear me now? 'Strong signal' from sun-like star sparks alien speculation Interesting... Wait though With only one signal you just can't tell, and it certainly doesn't come up to scratch as "Extraordinary claims require extraordinary evidence.". It might have been just the cleaner in the...
  6. Artlav

    I Is there a point during star formation when gas is 1 atm?

    I have been reading about and contemplating the early stages of star formation lately. An interstellar cloud collapses under it's own gravity to form a star. There is much data about what could trigger it, and what happens when the gas heats up or starts fusing. However, i couldn't find...
  7. Noisy Rhysling

    Star Trek-style crews: Logistical nightmare or not?

    We see many species of human/near-human on the Federation ships. It seems to me that there would be a logistics issue here. Things that are fine for humans might be poisonous for other species, and vice versa. Any idea how they would handle this? (Remember, Federation researchers will be...
  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. Khatti

    Building a Dyson Sphere around a Class-B Star

    I was idly thinking of Dyson Spheres and solar energy when this idea came to me: say you were technologically advanced enough to build a perfect, Dyson sphere and were able to deal with the problems of drift--in other words you belong to a really, technologically advanced civilization! It...
  10. Sebastiaan

    I Where is the dead star that created us?

    Alright, from my understanding we are created from star stuff, some of the atoms in our body were created during a Supernova. After a supernova, a neutron star should remain. So where is the remnant of Big Daddy, the star that seeded our solar system and therefore created us?
  11. G

    I Is the boundary of a star objective or subjective?

    What I mean by this question is the following: If, just for example, we define the surface region of a star as that where the matter undergoes a phase transition from plasma to radiation, then that boundary has an objective physical meaning (let's not bother with the fact that the transition is...
  12. D

    Density of star from hydrostatic equilibrium and pressure

    Homework Statement Assume that the pressure p in a star with spherical symmetry is related to the density \rho by the (distinctly unrealistic) equation of state p= \tfrac12 k\rho^2, where k is a constant. Use the fluid equilibrium equation obtained in Problem 23 to find a relation between \rho...
  13. wolram

    Is this the start of Star Trek gel bags

    Used in star trek as the cores for computers the gel bags are living cells, so is this a start for them to become a reality? https://www.sciencedaily.com/releases/2016/07/160721151225.htm
  14. GW150914

    A question about Star Trek's impulse drive

    We all know that starships in Star Trek have artificial gravity. They also have inertial damper. Therefore, we can conclude that they have the ability to counteract gravity. However, as those technical manuals of Star Trek tell us, starships are using impulse drives for slower-than-light...
  15. P

    I How can we measure a galaxy's velocity via a star?

    In this video (), we first took the spectrum of a star and then of a galaxy which look like the following: However, what I don't get is: if we take the spectrum of a star and then of the galaxy the star is in, shouldn't it give me the same spectrum as they're moving in the same speed? And if...
  16. P

    I How Can We Measure Redshift Using the Spectrum of a Star?

    As far as I know, when we use the spectrum of a star, we see where the absorption lines are and using this, we can detect the elements that are present in the star. We also measure whether those absorption lines are supposed to be for a particular element. But why is it not possible that the...
  17. P

    I Determine the temperature of a star via its spectrum

    In the following video (, diagram also given for reference), the professor says that if the graph peaks in a short wavelength, then the star is a hot star (or galaxy) and if it does so in a long wavelength, then it's a cold one. However, I fail to understand this. How does it happen that if a...
  18. GeorgeDishman

    I Neutron Star Stability: Speed of Sound vs Light

    [This thread was split from https://www.physicsforums.com/threads/a-light-wave-and-a-sound-wave-travelling-together.878450/#post-5518001 as it's an interesting topic in its own right] That's actually an important point, in the core of neutron stars, the speed of sound increases with the...
  19. TheQuietOne

    How Does Repulsorlift Technology Work in Star Wars?

    I am working on another star wars project but i need to know how a repulsorlift works ... any ideas?
  20. D

    Air leak on a Star Trek-style shuttle - what happens?

    Hi. I'm writing a space opera with your typical Star Trek-style shuttle. In the story, the ship is hit and begins leaking air. I want to knock out the characters (e.g., due to reduced oxygen before they can be rescued) without permanently injuring them due to loss of pressure or other effects...
  21. R

    How fast could the Emporer from Star Wars lightning be?

    Remember when he shot it at Mace Windu? How fast was that? Does it mean Mace Windu can react at the speed of ligfht?? What about when he shot Luke Skywraker with it? Or Hano Solo? basic Question?: How fast is star wars lightning?
  22. G

    Are Holodecks Possible in the Future of Star Trek?

    What happened to the Holodecks in the lastest Star Trek installments? A million years from now.. would it be possible to build Holodecks? Is it connected with whether realism is true or violated (such as in entanglement)? Because if realism or particles existing before measurements didn't...
  23. nsaspook

    Star Trek' actor Anton Yelchin dead in accident

    RIP http://jalopnik.com/anton-yelchins-jeep-was-included-in-rollaway-recall-lin-1782277820 http://www-odi.nhtsa.dot.gov/acms/cs/jaxrs/download/doc/UCM497024/INOA-EA16002-6630.PDF
  24. Janus

    Photos from visit to Star Trek exhibit.

    My wife and I drove up to Seattle recently to see this exhibit at the EMP celebrating 50 years of Star Trek. Here are some pictures of some of what they had on display. Kirk and McCoy's uniforms, Original captain's chair, and helm/navigation console from bridge.Uniform and props from Deep Space...
  25. K

    Moment of inertia of a neutron star

    Homework Statement Homework Equations rotational kinetic energy = 0.5 I ⋅ M ⋅ ω², where I is the moment of intertia, M the mass and ω the angular speed The Attempt at a Solution T = period of revolution K = kinetic energy associated with rotational moviment Since T increases with time...
  26. 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...
  27. C

    I What is the size of neutrons in a neutron star?

    Does the pressure within a neutron star compress the neutrons to a smaller size?
  28. Willfrid Somogyi

    What Species are "People" in Star Wars?

    One big plot hole seems to be the fact Star Wars is set a long time ago in a galaxy far, far away and yet somehow there are humans. Can anyone tell me whether they are actually humans or whether they just have an extremely uncanny resemblance but are in fact another species, is there concrete...
  29. T

    How to calculate the size of a star with just temperature

    Homework Statement This isn't a specific problem, but my professor told us that we can calculate the size of a star if we know the temperature of it. Homework Equations Stefan-Boltzmann Law, Wien's displacement law. The Attempt at a Solution With the temperature i can also get the intensity...
  30. T

    B At what point would we stop seeing a star?

    The universe is expanding and at some point faster than light so once it reaches that point we won't see the light from those stars. I forget what this point is called. If someone happened to be watching a star as it crossed that point would we see it simply blink out or close to that or would...
  31. G

    I Star Radius & Mass from Spectral Class, B-V, Luminosity

    I have a data set of 120k star systems that I'd like to import into a project, and, while it has a lot of useful infomation, I'd like to display these stars in a visual fashion. This means that I need to figure out the radius, when zoomed into the star system, and its mass, to simulate objects...
  32. GiantSheeps

    B What would happen if the Sun collided with a similar star?

    What would happen if the Sun collided with an exact copy of itself? I can't find any information online about this exact scenario. So what would happen to the Sun? What would happen to the Earth and the rest of the planets? Any information or help would be greatly appreciated. Thanks so much!
  33. tony873004

    B North & South Star: Polaris & Sirius

    Polaris won't always be the North Star. Year by year, it is currently getting closer to the North Celestial Pole. So for our lifetimes and the lifetimes of our children, Polaris will be the North Star. But things start to change for our great-great grandchildren. Over a 26,000 year period...
  34. TheQuietOne

    FTL Collisions in Star Wars: Fact or Fiction?

    I am a big Star Wars fan and I was just wondering what would happen if two Star Destroyers collided during lightspeed? could it be possible to go so fast you could go through something without destroying it?
  35. 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...
  36. P

    Estimate time scale of expansion of a star

    Homework Statement A star experiences a sudden increase in pressure to a new value P'. Taking the average density as ρ and the radius R, estimate the time scale of expansion of the star. (Ignore gravity altogether). Homework Equations The only thing I can see in my notes that may (or may not)...
  37. 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...
  38. Tom MS

    Falling to a Star With Varying Acceleration

    Take an object of negligible mass that is dropped from rest 2 kilometers away from a neutron star of mass 1.989*10^30 kilograms (1 solar mass) and radius 7,802 meters. How long will it take the object to reach the surface of the neutron star? I'm not terrible at calculus, but I know for a fact...
  39. K

    B How to calculate the distance between a Star and Zenith

    I was reading through some questions online and one asked the reader to calculate the distance between a star and Zenith given Sidereal Time 17hrs, RA*=16hr30mins, DEC*=50degrees. Could someone explain to me how you would do this please? There were no examples and so far I haven't managed to...
  40. Dotini

    B What is the source of the mysterious red rectangle in space?

    How charming! (Photo: ESA/Hubble and NASA) http://www.usatoday.com/story/tech/sciencefair/2016/04/08/star-red-rectangle-nasa-hubble/82796522/
  41. DennisN

    Who was the true hero behind the Rebel Alliance's victory in Rogue One?

    A trailer is out now:
  42. StanEvans

    I Neutron Star Formation: Quarks, Protons, Electrons Explained

    So a programme that I watched on tv was talking about neutron stars and they said that the neutron stars neutrons were formed by protons and electrons combining to make neutrons. I was just wondering, how does this work, in the field of the quarks in the proton and how they are effected by an...
  43. Stephanus

    B Lowest/Highest mass of a star, star system

    Dear PF Forum, My friend just tell me about Kelt - 4A. Kelt - 4A orbits 3 stars. I think such thing is not uncommon in the universe for binary/ternary systems. It's a matter of probability. Probability means that for any system there can be many celestial objects. So how many percent do they...
  44. AstroCody

    B Tidally Locked Planets: How Close to the Star?

    How close must a planet be to its parent star in order for it to be tidally locked?
  45. M

    B Einstein’s equivalence principle for a collapsed star

    The Equivalence Principle says depicting gravity as a field associated with matter is equivalent to picturing it as twisting of space. Is there a space-twisting equivalent of a collapsed star as a shell of condensed matter with interior of intense gravitational field energy (found in the field...
  46. CollinsArg

    About light -- why when I look at a star I see rays around it?

    Hi! I'm not studying physics and what I know about light is mostly what I've read on the internet and what I've learn thinking and observing the nature. I come to realize that the space is full of light. When I look at the black space, I don't see light but I know that is light everywhere up...
  47. D

    B The lifespan of a neutron star

    How durable are the neutron stars, ie how long can they last? Will they "evaporate" like black holes or something else will happen with them after a very long period of time (eons)? Or are they immortal? I'm asking because I have found nothing clear about the lifespan of a neutron star.
  48. wolram

    B Discover the Phenomenon of One New Star Every Hour: Science Daily"

    This from Science daily, they these galaxies are producing a star every hour, is that possible? https://www.sciencedaily.com/releases/2016/03/160322161651.htm
  49. FruitNinja

    Planet orbiting around a star whose mass changes

    Homework Statement (Assuming all circular orbits) [/B] Say there is a star with mass M and a planet orbiting that star with a mass m. The star M then suddenly loses half of its mass. (So now it is M/2) What is the new radius of orbit of the planet around the star? Warning: Velocity will not...
  50. Stella.Physics

    I Binary system of a neutron star with a black hole

    I am reading about mergers of a neutron star with a black hole and the association with GRB. While talking about the accretion disk, the paper I am reading says about the last stable circular orbit. Firstly I thought it was where the Roche limit lies, where mass transfer or matter ejection from...
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