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. H

    Precession of Earth's axis and alignment with stars

    I picked up a book about the pyramids at Cheops. (Relax, it wasn't a crackpot work.) It said that the pyramid was aligned with the star Polaris, and that Sirus and Orion may be sighted through some of the ventilation shafts. Now the pyramid is aligned with true north. It the time of the...
  2. Ken G

    Wrong Explanations for the Luminosity of Main-Sequence Stars

    There is a rather surprising thing afoot in many introductory astronomy texts, and many online web course notes, relating to a commonly seen but completely wrong explanation for why high-mass main-sequence stars have a much higher luminosity than low-mass main-sequence stars. You would really...
  3. L

    Kerr metric and rotating stars

    I have recently come across the notion that Kerr metric describes the spacetime outside a rotating black hole but not outside a rotating (electrically neutral) star. Unlike Schwarzschild metric, which works both for non-rotating spherically symetric black hole without charge as well as any other...
  4. L

    Differential equation for motion within stars

    I know: f = ma f = - GMm/r^2 a = -GM/r^2 can be easily derived, But, we've been given the differential equation of motion of gas within a star as: a = -GM/r^2 - 1/p * dP(r)/dr I was wondering where the - 1/p * dP(r)/dr term is derived from? I can't find it in my textbooks. Cheers
  5. Euclid Areti

    Gravity of planets, stars etc.

    Hello. I am by no means knowledgeable in General Relativity. With that said: What if "curvature of space-time" actually is the force fields of massive objects, like the magnetic field of earth? Why must space-time necessarily have to be "bent"? Is there really only one way to look at the nature...
  6. James William

    Planetary orbit disruption by passing stars

    I read this interesting paper showing that in less than a million years, one star will fly past our Sun at a distance of about 0.13 light years [1]. How might I calculate the maximum disruption this could cause on the orbit of an object in orbit around our Sun, such as planets and comets...
  7. L

    Binary Stars and Apparent Magnitudes

    Homework Statement A binary star system at a distance of 85pc consists of two stars of equal luminosity that are so close together that they are observed in a telescope as a single image. If the apparent magnitude of the two stars combined is 10.7 what would be the apparent magnitude of just...
  8. LachyP

    Forming Strange Quarks in Strange Stars

    I have done large amounts of research and could not find any books, sites or forums of how the strange matter in strange stars is formed. I know that the process of the formation of the strange star carries out mostly like the collapse into neutron stars and quark stars but how do these strange...
  9. D

    Creation of spinning neutron stars and pulsars

    I've gotten a bit confused about the creation of the neutron star/pulsar, so I was hoping someone could point me in the right direction :) As fusion stops, when reaching the iron phase, the outer layers (hydrogen, helium, carbon... etc.) gets pulled in-wards do to gravity. This creates a bounce...
  10. Kostas Tzim

    Birth of Stars: Deuterium, H & Li Isotopes

    Recently i watched a video in Khan academy about the birth of a star and i got kind of confused, a deuterium fuses with an H to create He right? then what happens if a He fuses with an H and form an isotope of Li ? When this sequence stops? ps: how can i apply Latex? i can't find the icon...
  11. S

    Measuring the Movement of the Stars

    I was wondering, since the Earth is constantly moving around the sun, the sun is moving around the milky way, and the milky way is moving through space (and for all I know the milky way might also be rotating around our local group, cluster, super cluster, etc.), how do we know how fast...
  12. T

    About neutron decay in neutron stars

    Hi, I was reading about free neutron beta decay the other day and it came to me the idea of neutron stars. As I understand, neutron stars are held together by gravity instead of strong force interaction (which I think is the mechanism that gives stability to neutron in common nuclei). So one...
  13. R

    Combinatorics help: stars and bars and beyond

    We have to distribute m distinguishable toys, k identical candy bars to 12 children in the following ways: a. How many ways can we distribute toys if each child can get any number of toys? b. How many ways can we distribute candy if each child can get any number of candy bars? c. How many ways...
  14. W

    How Much Declination Do Stars Need for a Luck City at Latitude 35°?

    People in the past thought that a city which the length of the visible arc of a star's path in that city is longer, that city is the luck city; Now, for a city with a latitude of 35° how much should be the star's declination to be the luck city?
  15. TheBiologist

    Do Some Stars Possess A Particular Name?

    Apart from the sun, do any other stars have a name? Just curious.
  16. J

    Why Stars Explode: Examining the Physics Behind Supernovas

    All of my internet research comes up with the same answer which is that when the core fuses all of its fuel, there is no more outward pressure, so the gravitational attraction takes over. What isn't explained is WHY it will explode outwards. I realize that the temperature will increase, but...
  17. V

    Stars & Electromagnetic Spectrum

    Hi Is the light from other stars has the same electromagnetic spectrum ?
  18. C

    Convection vs Radiation at the giant stage of stars

    I just read the following about the evolution of stars: "When reaching point 2 in the HR-diagram, the radius of the star has been increasing so much that the surface temperature is close to 2500 K which is a lower possible limit. When reaching this limit, the dominant mechanism of energy...
  19. marcus

    The Fastest Unbound Stars in the Universe

    http://arxiv.org/abs/1411.5022 The Fastest Unbound Stars in the Universe James Guillochon (1), Abraham Loeb (1) ((1) Harvard ITC) (Submitted on 18 Nov 2014) The discovery of hypervelocity stars (HVS) leaving our galaxy with speeds of nearly 103 km s−1 has provided strong evidence towards the...
  20. Iulian Ionescu

    Dyson slingshot & Neutron stars

    Hi, I am currently working on a Sci/Fi short story where a spaceship leaving a planet will perform a gravitational assist sling through a binary neutron star system. The problem is that I have difficulty finding information about how close a planet could be to a binary neutron star system? I...
  21. K

    How Did the First Stars Form Without Supernova Compression?

    I was watching Nova Science Now and they were talking about how stars form when gas clouds are compacted when hit by supernova shockwaves, which then allows them to compress further by their own gravity and eventually they ignite. But I was left wondering how then the first stars were able to...
  22. S

    Schonberg-Chandrasekhar mass for low mass stars

    Hi guys, I am dealing with the following issue: in low mass stars during the main sequence, the core gets filled with He ashes over time until the pressure is not sufficient to keep it in hydrostatic balance. Then the core starts to contract on KH timescale until it hits the full degeneracy...
  23. wolram

    Stars that look older than the universe

    For instance take Methuselah 140283, how can these stars exist?
  24. E

    Seeing stars during the daytime

    Is it possible to built a camera which filters blue light of the sky at the day, so that the stars can be visible?
  25. Spinnor

    "Churning galaxy boasts a fiery halo of baby stars"

    From, http://www.newscientist.com/article/dn26438-churning-galaxy-boasts-a-fiery-halo-of-baby-stars.html#.VEmn2iJ4pcQ
  26. C

    Which Stars Cast the Milky Way's First Light?

    by Dr. Ken Croswell Hubble spots a star that likely predates its oldest star clusters. Link at Scientific American.
  27. nicolauslamsiu

    Determine the least value of declination of stars so that stars can be seen?

    For stars to be observed in a certain place, the stars must be at a position above the horizon. Yet, how to determine the least value of declination of stars so that stars can be seen?
  28. avito009

    How Stars Ignite - The Fusion Reaction

    Stars are formed from large clouds of gas. The gas, which might be the wisps of older stars which have exploded, or simply left from the Big Bang, is very cold and the cloud begins large but not at all dense.Gravity causes all the atoms of the gas to pull each other together, makes the cloud...
  29. R

    Gravitation Redshift for very dense stars

    Homework Statement In deriving the expression ##\frac{f'-f}{f}=\frac{gH}{c^2}=\frac{GM_s}{R_sc^2}## , it was assumed that ##\triangle f=f'-f## was small, and that the photon had a constant mass of ##\frac{hf}{c^2}##. Suppose that a star is so dense that ##\triangle f## is not small. (a) Show...
  30. P

    Tension Shells & Tension Stars: Exploring the Israel Formalism

    I've been learning the Israel formalism (see original article here) for thin shells. I think I understand the formalism well and how to do the matching given two manifolds (that are solutions of the Einstein's field equations - EFE). I've been studying several articles that use the matching...
  31. Philosophaie

    List of Stars and Objects equating Names

    I am trying to create a list of stars. I need a comprehensive comparison between: FK5#s, HD#s and BD#s and NGC#s, IC#s and Proper Names (like Eagle Nebula) Here is a reference from FK5#s, HD#s and BD#s: http://www-kpno.kpno.noao.edu/Info/Caches/Catalogs/SAO/sao_catalog.html What...
  32. skujesco2014

    Relationship between Ultracold neutrons and neutron stars

    Hi, PF: I'm currently about to graduate from my Ph. D. program in Physics and I want to focus my research in theoretical physics. I feel very excited by topics such as Astrophysics and GR, but also low temperature physics, such as superconductivity, bose-einstein condensation, superfluidity...
  33. S

    Misalignment of angular momentum of stars (simulation)

    hey all! I have just finished processing the data of an n-body simulation (RAMSES code was used to do the simulation and AMIGA halo finder found our halos) ,so I have chosen a specific halo and it contains a massive galaxy (and of course satellite galaxies around it) and the code was run from...
  34. P

    Were the first stars much larger and shorter-lived than our sun?

    Looking at the huge clumps of higher elements (carbon silicon iron etc) present in our solar system, it seams as though only a few tens of billions of years would not be enough time to make all of the higher elements in the quantities we see today with stars like our sun that last billions of...
  35. S

    Radio Waves from Stars: Investigating the Mysteries

    dear sir i want to know what happens in certain stars that helps it give out radio wave we detect by radio telescope. can we compare with inductor in electronic circuit?
  36. C

    How Does the Velocity of Distant Stars Relate to the Mass of the Universe?

    Stars and galaxies that are further away from us are moving away faster, based on the red shift. What is the relationship between the maximum velocity of the furthest ones, as a fraction of the speed of light, and the mass of the universe? If we extrapolate the velocity up to the speed of light...
  37. M

    Exploring Time Dilation and Length Contraction with Alice's Spaceship

    This is my first post. :) Here it goes... Suppose alice's spaceship is traveling at speed close to c.. in line with a star. Because of length contraction she's going to find the distance towards it shorter than the "rest" distance. Does this mean she's going to get a closer look at the star...
  38. S

    Why does the furthest stars seem to be moving away faster than light?

    I heard somewhere that the amount of redshift in the most distant stars show that they are moving away faster than light and its due to the shape of the universe. I was wondering how do we even know what she shape of the universe is, I thought we couldn't see any boundaries.
  39. O

    Why do I see stars as a point in the sky?

    The light from stars radiates out from them as a sphere. When a part of the surface of the light sphere reaches the earth, it is far enough away from the star so that each point on that part of the surface of the sphere near the Earth has about the same light energy. But, I only see a point...
  40. K

    Why does the Sun appear to change color throughout the day?

    I've read previous threads that the color of our sun from outer space is white, but on Earth it is yellow because oxygen and nitrogen scatter the blue light. First, I am wondering if this is accurate. Second, if this is true, why do we see stars in the night sky as white? I would think the...
  41. S

    How to look at stars to go back in time?

    I understand the basic analogy of looking at things from the past. Let me explain what I know before I ask about what I want to know. The sun is 8 min light speed away. So whatever we see is the 8 min ago sun. Similarly, If we see the star 1 light year far...we can say we are looking at...
  42. S

    Stars Creating Elements Heavier than Iron

    I am curious how stars form elements heavier than iron. I have read that it generally occurs in the supernova death of a star or when neutron stars collide but have not read anything much more specific. I have read the wiki link on Nucleosynthesis but it doesn't have much in the way of...
  43. N

    Astrophysics (Stars - PP chain)

    Homework Statement Estimate the lifetime of a proton against fusion to 4He in the center of a Zero-Age-Main-Sequence solar mass star. First calculate the energy generation, εpp in the center of the star from the p-p chain. Then convert this to the number of fusions (conversion of 4 protons...
  44. T

    Two stars orbiting a common center

    Homework Statement Two stars, each of mass M, move in a circular orbit of radius R around their common center of mass C. a) what is the gravitational force on each star? b) what is the period of each star's orbit? c) what is the total mechanical energy (potential + kinetic) of this...
  45. jlefevre76

    Is Neutron Star Energy a Disappointment for Nuclear Renewables?

    So, I was disappointed to find out that the surface of a neutron star is at 1 million kelvin, not 100 billion kelvin. I did some calculations a while back using the 100 billion K as a temperature (the core temperature of a neutron star), and found that it would be emitting more radiation than...
  46. Ken G

    Understanding the Luminosity of Radiative Stars

    In the past I have tried to start threads that provide surprising insights into the processes that set the luminosity of main-sequence stars that have radiative diffusion as their primary energy transport mechanism (as opposed to fully convective stars). However I failed to include professional...
  47. Z

    Why do stars show absorption lines?

    On an intuitive level, Why do stars show absorption lines? For example, the 121.6nm photon is produced when an electron falls into the lowest level of a hydrogen atom. A 121.6nm photon is absorbed by a hydrogen atom being kicked into an excited state, where it will eventually emit a 121.6nm...
  48. S

    Main Sequence Star Mass: Star 1, 2 & 3 Comparison

    Homework Statement consider the information given below about three main sequence stars. Star 1 will be a main sequence star for 4.5 billion years. Star 2 has a spectral type of M5. Star 3 has the same luminosity as the Sun. Which has the greatest mass or are they approx. the same?Homework...
  49. S

    Nearest Stars to Earth: Red Dwarfs Dominant

    Why are the nearest stars to Earth mainly red dwarfs? When inspecting a list of approx. the 12 brightest stars vs. the nearest stars I see clear differences in luminosity. Again, what explains why these lists are so different in terms of spectral type?! Does it have to do with the amount of...
  50. F

    Stellar structure of stars problem

    Homework Statement Consider a star of radius R, with density p that is constant, composed of classical, nonrelativistic, idealg gas of fully ionized hydrogen. a. Solve the equations of stellar structure for the pressure profile, P(r) with the boundary condition P(R)=0 b. Find the temperature...
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