Stars | Science Mission Directorate (2023)

Stars are the most widely recognized astronomical objects, and represent the most fundamental building blocks of galaxies. The age, distribution, and composition of the stars in a galaxy trace the history, dynamics, and evolution of that galaxy. Moreover, stars are responsible for the manufacture and distribution of heavy elements such as carbon, nitrogen, and oxygen, and their characteristics are intimately tied to the characteristics of the planetary systems that may coalesce about them. Consequently, the study of the birth, life, and death of stars is central to the field of astronomy.

Star Formation

Stars are born within the clouds of dust and scattered throughout most galaxies. A familiar example of such as a dust cloud is the Orion Nebula. Turbulence deep within these clouds gives rise to knots with sufficient mass that the gas and dust can begin to collapse under its own gravitational attraction. As the cloud collapses, the material at the center begins to heat up. Known as a protostar, it is this hot core at the heart of the collapsing cloud that will one day become a star. Three-dimensional computer models of star formation predict that the spinning clouds of collapsing gas and dust may break up into two or three blobs; this would explain why the majority the stars in the Milky Way are paired or in groups of multiple stars.

Powerful Stellar Eruption
The observations of Eta Carinae's light echo are providing new insight into the behavior of powerful massive stars on the brink of detonation.
Credit: NOAO, AURA, NSF, and N. Smith (University of Arizona)

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As the cloud collapses, a dense, hot core forms and begins gathering dust and gas. Not all of this material ends up as part of a star — the remaining dust can become planets, asteroids, or comets or may remain as dust.

In some cases, the cloud may not collapse at a steady pace. In January 2004, an amateur astronomer, James McNeil, discovered a small nebula that appeared unexpectedly near the nebula Messier 78, in the constellation of Orion. When observers around the world pointed their instruments at McNeil's Nebula, they found something interesting — its brightness appears to vary. Observations with NASA's Chandra X-ray Observatory provided a likely explanation: the interaction between the young star's magnetic field and the surrounding gas causes episodic increases in brightness.

Main Sequence Stars

A star the size of our Sun requires about 50 million years to mature from the beginning of the collapse to adulthood. Our Sun will stay in this mature phase (on the main sequence as shown in the Hertzsprung-Russell Diagram) for approximately 10 billion years.

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Stars are fueled by the nuclear fusion of hydrogen to form helium deep in their interiors. The outflow of energy from the central regions of the star provides the pressure necessary to keep the star from collapsing under its own weight, and the energy by which it shines.

As shown in the Hertzsprung-Russell Diagram, Main Sequence stars span a wide range of luminosities and colors, and can be classified according to those characteristics. The smallest stars, known as red dwarfs, may contain as little as 10% the mass of the Sun and emit only 0.01% as much energy, glowing feebly at temperatures between 3000-4000K. Despite their diminutive nature, red dwarfs are by far the most numerous stars in the Universe and have lifespans of tens of billions of years.

On the other hand, the most massive stars, known as hypergiants, may be 100 or more times more massive than the Sun, and have surface temperatures of more than 30,000 K. Hypergiants emit hundreds of thousands of times more energy than the Sun, but have lifetimes of only a few million years. Although extreme stars such as these are believed to have been common in the early Universe, today they are extremely rare - the entire Milky Way galaxy contains only a handful of hypergiants.

Stars and Their Fates

In general, the larger a star, the shorter its life, although all but the most massive stars live for billions of years. When a star has fused all the hydrogen in its core, nuclear reactions cease. Deprived of the energy production needed to support it, the core begins to collapse into itself and becomes much hotter. Hydrogen is still available outside the core, so hydrogen fusion continues in a shell surrounding the core. The increasingly hot core also pushes the outer layers of the star outward, causing them to expand and cool, transforming the star into a red giant.

If the star is sufficiently massive, the collapsing core may become hot enough to support more exotic nuclear reactions that consume helium and produce a variety of heavier elements up to iron. However, such reactions offer only a temporary reprieve. Gradually, the star's internal nuclear fires become increasingly unstable - sometimes burning furiously, other times dying down. These variations cause the star to pulsate and throw off its outer layers, enshrouding itself in a cocoon of gas and dust. What happens next depends on the size of the core.

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Average Stars Become White Dwarfs
For average stars like the Sun, the process of ejecting its outer layers continues until the stellar core is exposed. This dead, but still ferociously hot stellar cinder is called a White Dwarf. White dwarfs, which are roughly the size of our Earth despite containing the mass of a star, once puzzled astronomers - why didn't they collapse further? What force supported the mass of the core? Quantum mechanics provided the explanation. Pressure from fast moving electrons keeps these stars from collapsing. The more massive the core, the denser the white dwarf that is formed. Thus, the smaller a white dwarf is in diameter, the larger it is in mass! These paradoxical stars are very common - our own Sun will be a white dwarf billions of years from now. White dwarfs are intrinsically very faint because they are so small and, lacking a source of energy production, they fade into oblivion as they gradually cool down.

This fate awaits only those stars with a mass up to about 1.4 times the mass of our Sun. Above that mass, electron pressure cannot support the core against further collapse. Such stars suffer a different fate as described below.

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White Dwarfs May Become Novae
If a white dwarf forms in a binary or multiple star system, it may experience a more eventful demise as a nova. Nova is Latin for "new" - novae were once thought to be new stars. Today, we understand that they are in fact, very old stars - white dwarfs. If a white dwarf is close enough to a companion star, its gravity may drag matter - mostly hydrogen - from the outer layers of that star onto itself, building up its surface layer. When enough hydrogen has accumulated on the surface, a burst of nuclear fusion occurs, causing the white dwarf to brighten substantially and expel the remaining material. Within a few days, the glow subsides and the cycle starts again. Sometimes, particularly massive white dwarfs (those near the 1.4 solar mass limit mentioned above) may accrete so much mass in the manner that they collapse and explode completely, becoming what is known as a supernova.
Supernovae Leave Behind Neutron Stars or Black Holes
Main sequence stars over eight solar masses are destined to die in a titanic explosion called a supernova. A supernova is not merely a bigger nova. In a nova, only the star's surface explodes. In a supernova, the star's core collapses and then explodes. In massive stars, a complex series of nuclear reactions leads to the production of iron in the core. Having achieved iron, the star has wrung all the energy it can out of nuclear fusion - fusion reactions that form elements heavier than iron actually consume energy rather than produce it. The star no longer has any way to support its own mass, and the iron core collapses. In just a matter of seconds the core shrinks from roughly 5000 miles across to just a dozen, and the temperature spikes 100 billion degrees or more. The outer layers of the star initially begin to collapse along with the core, but rebound with the enormous release of energy and are thrown violently outward. Supernovae release an almost unimaginable amount of energy. For a period of days to weeks, a supernova may outshine an entire galaxy. Likewise, all the naturally occurring elements and a rich array of subatomic particles are produced in these explosions. On average, a supernova explosion occurs about once every hundred years in the typical galaxy. About 25 to 50 supernovae are discovered each year in other galaxies, but most are too far away to be seen without a telescope.

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Neutron Stars
If the collapsing stellar core at the center of a supernova contains between about 1.4 and 3 solar masses, the collapse continues until electrons and protons combine to form neutrons, producing a neutron star. Neutron stars are incredibly dense - similar to the density of an atomic nucleus. Because it contains so much mass packed into such a small volume, the gravitation at the surface of a neutron star is immense. Like the White Dwarf stars above, if a neutron star forms in a multiple star system it can accrete gas by stripping it off any nearby companions. The Rossi X-Ray Timing Explorer has captured telltale X-Ray emissions of gas swirling just a few miles from the surface of a neutron star.

Neutron stars also have powerful magnetic fields which can accelerate atomic particles around its magnetic poles producing powerful beams of radiation. Those beams sweep around like massive searchlight beams as the star rotates. If such a beam is oriented so that it periodically points toward the Earth, we observe it as regular pulses of radiation that occur whenever the magnetic pole sweeps past the line of sight. In this case, the neutron star is known as a pulsar.

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Black Holes
If the collapsed stellar core is larger than three solar masses, it collapses completely to form a black hole: an infinitely dense object whose gravity is so strong that nothing can escape its immediate proximity, not even light. Since photons are what our instruments are designed to see, black holes can only be detected indirectly. Indirect observations are possible because the gravitational field of a black hole is so powerful that any nearby material - often the outer layers of a companion star - is caught up and dragged in. As matter spirals into a black hole, it forms a disk that is heated to enormous temperatures, emitting copious quantities of X-rays and Gamma-rays that indicate the presence of the underlying hidden companion.

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From the Remains, New Stars Arise
The dust and debris left behind by novae and supernovae eventually blend with the surrounding interstellar gas and dust, enriching it with the heavy elements and chemical compounds produced during stellar death. Eventually, those materials are recycled, providing the building blocks for a new generation of stars and accompanying planetary systems.

Recent Discoveries

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May 5, 2022Hubble Reveals Surviving Companion Star in Aftermath of Supernova
March 30, 2022Record Broken: Hubble Spots Farthest Star Ever Seen
March 14, 2022Tiny Star Unleashes Gargantuan Beam of Matter and Antimatter (PSR J2030+4415)
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March 7, 2022Hubble Snaps a Jet Set
March 1, 2022NASA’s NuSTAR Makes Illuminating Discoveries With ‘Nuisance’ Light
February 28, 2022The Unfolding Story of a Kilonova Told in X-rays (GW170817)
January 29, 2022Hubble Examines a Star-Forming Chamaeleon
January 25, 2022Visualization Explores a Massive Star's Great Eruption
January 12, 20221,000-Light-Year-Wide Bubble Surrounding Earth Is Source of All Nearby, Young Stars
November 23, 2021Hubble Finds Flame Nebula's Searing Stars May Halt Planet Formation
November 17, 2021Hubble Spies Newly Forming Star Incubating in IC 2631
November 16, 2021Nebula Churns Out Massive Stars in New Hubble Image
November 15, 2021SOFIA Observes Star Formation Near the Galactic Center
November 8, 2021Hubble Spots Dark Star-Hatching frEGGs
November 2, 2021Mysterious “Superbubble” Hollows Out Nebula in New Hubble Image
October 28, 2021Hubble Celebrates Halloween With A Glowering Carbon Star
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October 12, 2021When a Stable Star Explodes (G344.7-0.1)
September 22, 2021Hubble Finds Early, Massive Galaxies Running on Empty
September 6, 2021Hubble Discovers Hydrogen-Burning White Dwarfs Enjoying Slow Aging
August 31, 2021An Accidental Discovery Hints at a Hidden Population of Cosmic Objects
August 30, 2021Astronomy in Action (HH 111)
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August 4, 2021TESS Tunes into an All-sky ‘Symphony’ of Red Giant Stars
August 4, 2021NuSTAR and XMM-Newton See Light Echo from Behind a Black Hole
August 4, 2021Stars Are Exploding in Dusty Galaxies. We Just Can’t Always See Them
July 26, 2021Fermi Spots a Supernova’s ‘Fizzled’ Gamma-ray Burst
July 6, 2021SOFIA Witnesses Rare Accretion Flare on Massive Protostar


How long does it take for a star to form? ›

The process of star formation takes around a million years from the time the initial gas cloud starts to collapse until the star is created and shines like the Sun. The leftover material from the star's birth is used to create planets and other objects that orbit the central star.

How many black hole in Milky Way? ›

Most stellar black holes, however, are very difficult to detect. Judging from the number of stars large enough to produce such black holes, however, scientists estimate that there are as many as ten million to a billion such black holes in the Milky Way alone.

Are there black holes at the center of all galaxies? ›

Observational evidence indicates that almost every large galaxy has a supermassive black hole at its center. For example, the Milky Way has a supermassive black hole in its Galactic Center, corresponding to the radio source Sagittarius A*.

How star born? ›

Star Formation

Stars are born within the clouds of dust and scattered throughout most galaxies. A familiar example of such as a dust cloud is the Orion Nebula. Turbulence deep within these clouds gives rise to knots with sufficient mass that the gas and dust can begin to collapse under its own gravitational attraction.

What happens when a star dies? ›

When the helium fuel runs out, the core will expand and cool. The upper layers will expand and eject material that will collect around the dying star to form a planetary nebula. Finally, the core will cool into a white dwarf and then eventually into a black dwarf. This entire process will take a few billion years.

Can you leave a galaxy? ›

So, to leave our Galaxy, we would have to travel about 500 light-years vertically, or about 25,000 light-years away from the galactic centre. We'd need to go much further to escape the 'halo' of diffuse gas, old stars and globular clusters that surrounds the Milky Way's stellar disk.

Is there a real picture of the Milky Way galaxy? ›

It takes 250 million years for our Sun and the solar system to go all the way around the center of the Milky Way. We can only take pictures of the Milky Way from inside the galaxy, which means we don't have an image of the Milky Way as a whole.

Why is space black? ›

Because space is a near-perfect vacuum — meaning it has exceedingly few particles — there's virtually nothing in the space between stars and planets to scatter light to our eyes. And with no light reaching the eyes, they see black.

Where is the closest black hole to Earth? ›

In 2020 a team led by European Southern Observatory (ESO) astronomers reported the closest black hole to Earth, located just 1000 light-years away in the HR 6819 system.

Is there a real picture of a black hole? ›

The Event Horizon Telescope (EHT) Collaboration has created a single image (top frame) of the supermassive black hole at the centre of our galaxy, called Sagittarius A* (or Sgr A* for short), by combining images extracted from the EHT observations.

Will black holes reach Earth? ›

Despite their abundance, there is no reason to panic: black holes will not devour Earth nor the Universe. It is incredibly unlikely that Earth would ever fall into a black hole. This is because, at a distance, their gravitational pull is no more compelling than a star of the same mass.

What star is D? ›

Stars and star systems (D)
Star or star systemLocational references
Delta Pegasi (Alpha Andromedae)Andromeda constellation, visible from Sol (97 light-years)
Delta Sigma
Delta Triciatu
Deneb Kaitos
16 more rows

What is the death of a star called? ›

When a high-mass star has no hydrogen left to burn, it expands and becomes a red supergiant. While most stars quietly fade away, the supergiants destroy themselves in a huge explosion, called a supernova. The death of massive stars can trigger the birth of other stars.

How old is the Sun? ›

What is star life cycle? ›

Aside from our sun, the dots of light we see in the sky are all light-years from Earth. Massive stars transform into supernovae, neutron stars and black holes while average stars just like the sun, end life as a white dwarf star surrounded by a disappearing nebula.

What is the last stage of a star? ›

A planetary nebula is the final stage of a Sun-like star. As such, planetary nebulas allow us a glimpse into the future of our own solar system. A star like our Sun will, at the end of its life, transform into a red giant. Stars are sustained by the nuclear fusion that occurs in their core, which creates energy.

What is a average star? ›

An average star, or intermediate-mass star, is a star with an initial mass of 0.5 to 8 times that of Earth's sun. It spends most of its time on the main sequence as an orange, yellow, or blue-white dwarf star.

Do stars live forever? ›

Answer: No. Stars are born, live, and die. This process is called the "life cycle of a star".

How long is the life of a star? ›

Generally, the more massive the star, the faster it burns up its fuel supply, and the shorter its life. The most massive stars can burn out and explode in a supernova after only a few million years of fusion. A star with a mass like the Sun, on the other hand, can continue fusing hydrogen for about 10 billion years.

Is our Sun a star? ›

Our Sun is an ordinary star, just one among hundreds of billions of stars in the Milky Way Galaxy. As the only star we can observe in detail, it provides a basis for our understanding of all stars. The Sun is composed almost entirely of hydrogen and helium gas.

How fast is a star born? ›

If we turn this around, this mean that throughout the entire Universe, a star is born every 0.0002 seconds (i.e. every 2, 10,000th's of a second)!!

How long does it take to form a low mass star? ›

After about 100 million years, the star fuses all its core helium into carbon. Then a helium fusion shell forms around this core, and the hydrogen fusion shell remains around that.

Are stars still forming? ›

There are new Stars Forming Near the Core of the Milky Way Despite the Harsh Environment. The central core of our galaxy is not a friendly place for star formation, and yet new observations have revealed almost four dozen newly-forming systems.

How long does it take for a galaxy to form? ›

Galaxies evolve not only by merging with other galaxies, but also through 'internal' processes. This evolution is called secular, as it typically takes much longer than the external processes. Secular processes take a few billion years, while external processes take a few 100 million years.

What star is D? ›

Stars and star systems (D)
Star or star systemLocational references
Delta Pegasi (Alpha Andromedae)Andromeda constellation, visible from Sol (97 light-years)
Delta Sigma
Delta Triciatu
Deneb Kaitos
16 more rows

How many stars are dying? ›

We estimate at about 100 billion the number of galaxies in the observable Universe, therefore there are about 100 billion stars being born and dying each year, which corresponds to about 275 million per day, in the whole observable Universe.

What is the death of a star called? ›

When a high-mass star has no hydrogen left to burn, it expands and becomes a red supergiant. While most stars quietly fade away, the supergiants destroy themselves in a huge explosion, called a supernova. The death of massive stars can trigger the birth of other stars.

Do low mass stars become red giants? ›

Over time, massive stars become red supergiants, and lower-mass stars like the Sun become red giants.

How big are red giants? ›

Red giant stars bloat to 62 million to 620 million miles in diameter (100 million to 1 billion kilometers) — 100 to 1,000 times wider than our sun is today.

Do low mass stars burn oxygen? ›

Low mass stars end up as White Dwarfs composed of mainly Carbon and Oxygen. Medium mass stars have higher temperatures in their cores. The higher T allows fusion reactions creating Oxygen, Neon, Sodium and Magnesium. Medium mass stars end up as White Dwarfs composed of the higher mass elements.

How old is the Sun? ›

Is our Milky Way moving? ›

The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang.

Are new suns born? ›

We can wonder how our galaxy can contain 100 to 200 billion suns when the Universe is 13.8 billion years old: with a rate of 3 new suns born every year, the figure is far too high!

How old is our galaxy? ›

What are the 3 types of galaxies? ›

Astronomers classify galaxies into three major categories: elliptical, spiral and irregular. These galaxies span a wide range of sizes, from dwarf galaxies containing as few as 100 million stars to giant galaxies with more than a trillion stars.

Who created the galaxies? ›

Over billions of years, gravity caused gas and dust to form galaxies, stars , planets, and more. The matter that spread out from the Big Bang developed into everything in the universe, including you. You are made of star stuff! Astronomers have figured out that the universe is about 13 billion years old.


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