Stars are the most basic building blocks of galaxies.
The age, distribution, and composition of stars trace the history, dynamics, and evolution of their galaxy. Stars are responsible for the production and distribution of heavy elements, such as carbon, nitrogen, and oxygen.
Different star types have different habitable zones. This is the area around a star where conditions are just right, neither too hot nor too cold for liquid water to exist on a planet’s surface. (For this reason, a star's habitable zone is often referred to informally as its "Goldilocks zone.")
Statistically, there should be more than 100 billion planets in our Milky Way galaxy. They come in a wide range of sizes and characteristics. Complex organisms arose on Earth only 500 million years ago, and modern humans have been here for only 200,000 years – the blink of an eye on cosmological timescales. Earth will become uninhabitable for higher forms of life in a little over 1 billion years, as the Sun grows warmer and dries our planet. Therefore, stars slightly cooler than our Sun – called orange dwarfs – are considered better for advanced life. They can burn steadily for tens of billions of years. This opens up a vast timescape for biological evolution to pursue an infinity of experiments for yielding robust life forms. And, for every star like our Sun, there are three times as many orange dwarfs in the Milky Way.
The even more abundant star type called red dwarfs (also known as M dwarf stars) have even longer lifetimes. Planets in a red dwarf's comparatively narrow habitable zone, which is very close to the star, are exposed to extreme levels of X-ray and ultraviolet radiation, which can be hundreds of thousands of times more intense than what Earth receives from the Sun. Planets in the habitable zones of red dwarfs can be baked bone dry and have their atmospheres stripped away quite early in their lives. Red dwarfs typically calm down after a few billion years, but their early outbursts could prohibit their planets from evolving to be more hospitable.
How are stars born?
Stars are born from vast clouds of gas and dust, known as nebulae, that are scattered throughout most galaxies. Over thousands to millions of years, gravity can cause denser pockets within a nebula to collapse under their own weight. As a cloud – which is mostly hydrogen – collapses, the material at its center begins to heat up. Known as a “protostar,” this hot core of the collapsing cloud will is a star in the making. Some of these spinning clouds of collapsing gas and dust break up into two or three blobs that each form stars. This would explain why most of the stars in the Milky Way come in pairs or in multiples. Not all of this material ends up as part of the star, however – the remaining dust can become planets and moons, asteroids, and comets – or may simply remain as dust.
What is a main sequence star?
As millions of years pass, the core temperature of a protostar reaches a point at which nuclear fusion can begin. The star then begins the longest stage of its life, called the “main sequence.” Most stars in the galaxy, including our Sun, are categorized as main sequence. This is a state in which nuclear fusion in the star is stable and hydrogen is converted to helium. This process releases a lot of energy that keeps the star hot and bright, and it supplies an outward pressure against the incredible mass of material that would otherwise cause the star to collapse on itself. Ninety percent of a star’s life is spent in the main sequence phase.
What does a star's color mean?
What does a star's color mean?
When you look at the night sky, you may notice that some stars shine more brightly than others. The brightness of a star is related to how much energy it puts out, as well as how close it is to Earth.
Stars also vary in color – because they vary in temperature. Hotter stars appear blue or white, while cooler stars look orange or red. Astronomers use these characteristics to classify main sequence stars into categories by color and temperature: O (blue), B (blue-white), A (white), F (yellow-white), G (yellow), K (orange), and M (red), from hottest and biggest to coolest and smallest. Stars at the ends of their lives are out of the main sequence. These include supergiants, red giants, and white dwarfs.
What kind of star is our Sun?
Cool star, warm planet
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Our Sun is categorized as a G-type yellow-dwarf main sequence star. It is predicted that our Sun will remain in the main sequence phase for a few billion more years.
Stars can live for billions of years, but their lives can be shorter or longer depending on their size (technically, their mass). The bigger (or more massive) the star, the shorter its life, as more massive stars burn their nuclear fuel faster.
How do planets form around stars?
How do planets form around stars?
The gas and dust that swirl around a star during its formation are critical to forming planets around it. The dust contains heavy elements such as carbon and iron that form the cores of planets.
Stirred, not shaken: How pebbles become planets
Scientists think planets start off as grains of dust smaller than the width of a human hair. They emerge from the giant, donut-shaped disk of gas and dust that circles young stars. Gravity and other forces cause material within the disk to collide. If the collision is gentle enough, the material fuses, growing like rolling snowballs. Over time, dust particles combine to form pebbles, which evolve into mile-sized rocks. As these planetesimals, or planets in the making, orbit their star, they clear material from their path, leaving tracks of space empty but for fine dust. At the same time, the star gobbles up nearby gas while pushing more distant material farther away. After a few million years, the disk will have totally transformed, much of it taking the form of new worlds.
The life cycles of stars
Red giants and white dwarfs
When an average star like our Sun runs out of hydrogen to fuse, the star starts to collapse. But compacting a star causes it to heat up again and it is able to fuse what little hydrogen remains in a shell wrapped around its core. This burning shell of hydrogen greatly expands the outer layers of the star. When this happens, the star becomes a red giant. When our Sun enters the red giant phase of its life, in about 5 billion years, it will be so big that Mercury will be completely swallowed.
Our red giant Sun will still be consuming helium and cranking out carbon. When the helium is gone, the Sun will succumb to gravity again. When the core contracts, it will cause a release of energy and the Sun will become an even bigger giant with a radius beyond Earth's orbit.
After about a billion years as a red giant, the Sun will have ejected its outer layers until, eventually, its stellar core is exposed. This dead (in terms of nuclear fusion) but still ferociously hot stellar cinder is called a white dwarf. White dwarfs are roughly the size of Earth, despite containing the mass of a star. Pressure from fast moving electrons keeps these stars from further collapse. 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! White dwarfs fade into oblivion over many billions of years 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.
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 in the act of being born. Today, we understand that they are 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 on its surface. When enough hydrogen has accumulated on the surface, a burst of nuclear fusion erupts, causing the white dwarf to brighten substantially and eject its 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) may accrete so much mass in this manner that they collapse and explode completely, becoming what is known as a supernova.
Going supernova
Stars more than eight times the mass of our Sun 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. 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 5,000 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.
Neutron stars and pulsars
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. Because it contains so much mass packed into such a small volume, the gravity at the surface of a neutron star is immense. Like white dwarfs, if a neutron star forms in a multiple star system it can accrete gas by stripping it from nearby companions.
Neutron stars also have powerful magnetic fields that can accelerate atomic particles around its magnetic poles, producing powerful beams of radiation. Those beams sweep around like massive searchlights as the star rotates. If such a beam is oriented so that it periodically points toward Earth, we observe it as regular pulses of radiation that occur whenever the magnetic pole sweeps past our line of sight. In this case, the neutron star is known as a pulsar.
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, not even light.
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What Is a Black Hole?
Because 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, called an accretion 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.
Black holes that are quiet and not actively "feeding" on accretion disks can also be detected indirectly by observing the motions of nearby stars. For example, astronomers observe the supermassive black hole at the center of the Milky Way by watching as nearby stars whip around at astounding speeds only possible under the influence of an incredibly massive, but invisible object.
From the remains, new stars and planets arise
The dust and debris left behind by novae and supernovae, as well as by red giants puffing off their outer layers, eventually blend with the surrounding interstellar gas and dust, forming new nebulae. The products created in the ends of the lives of stars enrich galaxies with heavy elements and chemical compounds. Eventually, those materials are recycled, providing the building blocks for new generations of stars and planetary systems.
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FAQs
Stars | What is an Exoplanet? – Exoplanet Exploration: Planets Beyond our Solar System? ›
Most of the stars in our galaxy are thought to host their own families of planets. Thousands of these exoplanets have been discovered so far, with thousands more candidates detected and awaiting confirmation.
Are there planets around stars beyond our solar system? ›Most of the stars in our galaxy are thought to host their own families of planets. Thousands of these exoplanets have been discovered so far, with thousands more candidates detected and awaiting confirmation.
Is a exoplanet a star? ›An exoplanet is any planet beyond our solar system. Most orbit other stars, but free-floating exoplanets, called rogue planets, orbit the galactic center and are untethered to any star.
What stars have exoplanets? ›The stars with the most confirmed planets are Sol (the Solar System's star, also referred to as the Sun) and Kepler-90 with 8 confirmed planets each, followed by TRAPPIST-1 with 7 planets.
What is the difference between exoplanets and stars? ›Planets that orbit around other stars are called exoplanets. All of the planets in our solar system orbit around the Sun. Planets that orbit around other stars are called exoplanets. Exoplanets are very hard to see directly with telescopes.
What is beyond the solar system? ›Exoplanets are planets beyond our own solar system. Thousands have been discovered in the past two decades, mostly with NASA's Kepler Space Telescope. These worlds come in a huge variety of sizes and orbits. Some are gigantic planets hugging close to their parent stars; others are icy, some rocky.
What is beyond our universe? ›The trite answer is that both space and time were created at the big bang about 14 billion years ago, so there is nothing beyond the universe. However, much of the universe exists beyond the observable universe, which is maybe about 90 billion light years across.
Are all planets a star? ›Stars consist of matter like Hydrogen, Helium, and other light elements. Planets, on the other hand, contain solids, liquids, gases, or a combination thereon. Thus, this is the basic difference between stars and planets.
Is the Earth A star? ›Third Rock. Earth orbits our Sun, a star. Earth is the third planet from the Sun at a distance of about 93 million miles (150 million km).
Is Earth an exoplanet? ›Answer and Explanation: No earth is not an exoplanet but a terrestrial planet.
Where do stars come from? ›
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.
How many planets can a star system have? ›This limits the number of concentric orbital planes you can have around a star before a system becomes unstable. He found that, theoretically, a Sun-like star could support 42 planets in a single orbital plane. Around our own Sun, you could have six rings of 42 in the habitable zone, giving us 252 planets.
How many exoplanets are there per star? ›However, based on various studies, an average of between one and two exoplanets per star seems to be the most likely answer, giving as many as 400 billion planets in our Galaxy.
What are stars made of? ›A star is a huge glowing ball of hot gas. Deep inside its core, hydrogen atoms smash together, forming helium and releasing huge amounts of energy that heats the gas. This is called nuclear fusion, and it's why a star shines. As the hot gas pushes outward, it opposes the inward pull of gravity.
What are the 3 types of exoplanets? ›So far scientists have categorized exoplanets into the following types: Gas giant, Neptunian, super-Earth and terrestrial.
Do stars move around the Earth? ›These apparent star tracks are in fact not due to the stars moving, but to the rotational motion of the Earth. As the Earth rotates with an axis that is pointed in the direction of the North Star, stars appear to move from east to west in the sky.
What universe are we in? ›The Sun is one among hundreds of billions of stars in the Milky Way galaxy, and most of those stars have their own planets, known as exoplanets. The Milky Way is but one of billions of galaxies in the observable universe — all of them, including our own, are thought to have supermassive black holes at their centers.
Have we explored beyond our solar system? ›Astronomers have discovered two planets using NASA's Kepler telescope: a super Earth inferno and its Neptune-like companion. The knowledge and tools NASA has developed to study life on Earth will be a great asset to the study of planets beyond our solar system.
What's past our galaxy? ›Beyond our galaxy itself, which holds our solar system and everything beyond it, are other galaxies. The nearest galaxy is Andromeda, which will collide with the Milky Way galaxy in about 4 billion years. The Milky Way is part of the Local Group, which consists of 30 galaxies.
What was there before the universe? ›In the beginning, there was an infinitely dense, tiny ball of matter. Then, it all went bang, giving rise to the atoms, molecules, stars and galaxies we see today. Or at least, that's what we've been told by physicists for the past several decades.
Are we in a void universe? ›
Our Galaxy Is Also Surrounded By A Void
Not only is the inside of the Milky Way home to a big void, but chances are we're also surrounded by one. This is known as a Local Void, and likely surrounds the outside of the Milky Way galaxy. However, our galaxy tends to move towards areas with more density.
Even though certain features of the universe seem to require the existence of a multiverse, nothing has been directly observed that suggests it actually exists. So far, the evidence supporting the idea of a multiverse is purely theoretical, and in some cases, philosophical.
Have we found a star without a planet? ›After viewing more than 100,000 stars for years at a time, looking for planetary transits, the Kepler mission reached a startling conclusion: practically all stars have at least one planet.
What makes a star not a planet? ›Unlike stars, planets do not experience nuclear fusion, the process of combining tiny particles called atoms to release energy. Nuclear fusion creates radiation (heat and light) and makes stars glow. Because planets do not have nuclear fusion, they do not produce their own light.
What is a star but not a planet? ›Stars are the astronomical objects, that emit their own light, produced due to thermonuclear fusion, occurring at its core. Planets refers to the celestial object that has a fixed path (orbit), in which it moves around the star. Light. They have their own light. They do not have their own light.
Are we living on a star? ›Humans cannot live on a star because a star is too hot to support organisms (living things). Also because a star has no oxygen, H20 (water), or food.
Did Earth form from a star? ›The Earth formed over 4.6 billion years ago out of a mixture of dust and gas around the young sun. It grew larger thanks to countless collisions between dust particles, asteroids, and other growing planets, including one last giant impact that threw enough rock, gas, and dust into space to form the moon.
What is the real name of Earth? ›The real name for the Earth is Earth although it is occasional called Terra. Actually the real name is Terra. One can capitalize it when talking about the planet but when talking about... In several modern Romance languages, Terra or Terre is the name of planet Earth.
How many Earths are there in the universe? ›Out of those 40 billion Earth-like planets, how many other worlds might there be that support life? These same scientists have concluded that planets like Earth are relatively common throughout the Milky Way galaxy. In fact, the nearest one could be as close as about 12 light years away.
Will humans ever live on an exoplanet? ›Finding the Perfect Planet
In recent years, scientists have discovered a large amount of exoplanets, but close to none of them could actually support human life. For example, Kepler 10b, an exoplanet in another solar system, is close to the size of Earth, but it's too close to its star for human life.
How many super-Earths are there? ›
To date, astronomers have discovered two dozen super-Earth exoplanets that are, if not the best of all possible worlds, theoretically more habitable than Earth.
How did humans come from stars? ›Most of the elements of our bodies were formed in stars over the course of billions of years and multiple star lifetimes. However, it's also possible that some of our hydrogen (which makes up roughly 9.5% of our bodies) and lithium, which our body contains in very tiny trace amounts, originated from the Big Bang.
What happens when a star dies? ›The death of a larger star, for example, around 10 times as bright as the sun, results in a supernova explosion, the biggest explosion we humans have ever seen. And supernova explosions happen in a matter of seconds.
Are all stars in galaxies? ›Answer: No, not all stars are in a galaxy. They may have once belonged to a galaxy, but they are not a part of it any more. Some of these so-called "stellar outcasts" or "intergalactic stars" have actually been observed by NASA's Hubble Space Telescope.
How many galaxies are there? ›One such estimate says that there are between 100 and 200 billion galaxies in the observable universe. Other astronomers have tried to estimate the number of 'missed' galaxies in previous studies and come up with a total number of 2 trillion galaxies in the universe.
Why do we have stars? ›When it was dark these stars would light up the sky giving people light . In addition stars are very important because they make life on Earth. the most important is the Sun, because without that it wouldn't be life on Earth . Earth would just be a rock with ice.
Can a star be bigger than a solar system? ›And there are stars that are far more impressive. Blue supergiants can reach sizes 1,000 times larger than the Sun. This means that, if one were in the center of our solar system, it would almost be wide enough to span Jupiter's orbit (in essence, it would eat nearly our entire solar system).
What is the farthest known star? ›Located some 28 billion light-years away (thanks to the expanding universe), this 12.9-billion-year-old star, named Earendel, is between 50 and 500 times as massive as the Sun — and millions of times as bright.
What is the next habitable planet? ›On 24 August 2016, the team led by Anglada-Escudé proposed that a terrestrial exoplanet in the habitable zone of Proxima Centauri could explain these anomalies and announced Proxima Centauri b's discovery. In 2022, another planet named Proxima Centauri d, which orbits even closer to the star, was confirmed.
How many stars in the universe? ›Using the Milky Way as our model, we can multiply the number of stars in a typical galaxy (100 billion) by the number of galaxies in the universe (2 trillion). The answer is an absolutely astounding number. There are approximately 200 billion trillion stars in the universe. Or, to put it another way, 200 sextillion.
How long do stars live? ›
Giant stars use up their hydrogen fuel quickly, resulting in short lifetimes. An eight solar mass star will live less than 100 million years. At 10-15 solar masses, the lifetime of the star drops to only 10-20 million years. The most massive giant stars are believed to live no more than a few million years.
Do stars contain diamonds? ›High pressure experiments suggest large amounts of diamonds are formed from methane on the ice giant planets Uranus and Neptune, while some planets in other planetary systems may be almost pure diamond. Diamonds are also found in stars and may have been the first mineral ever to have formed.
Is Moon a star or not? ›The star which is the formation of hot gasses, energy, light and heat, which doesn't refer to the moon's qualities. So the moon is not a planet or a star.
How big is the universe? ›While the spatial size of the entire universe is unknown, it is possible to measure the size of the observable universe, which is approximately 93 billion light-years in diameter at the present day.
What is the super planet? ›Super-Earths – a class of planets unlike any in our solar system – are more massive than Earth yet lighter than ice giants like Neptune and Uranus, and can be made of gas, rock or a combination of both. They are between twice the size of Earth and up to 10 times its mass.
What is the largest planet in the universe? ›What's the biggest planet in the universe? The biggest planet in the known universe is likely a giant exoplanet named ROXs 42 Bb. This gas giant, nearly 500 light-years from Earth, was discovered in 2013. According to NASA, ROXs 42 Bb has a radius that's estimated to be 1.12 times that of Jupiter's.
What is the closest star to Earth? ›Proxima Centauri, the closest star to our own, is still 40,208,000,000,000 km away. (Or about 268,770 AU.) When we talk about the distances to the stars, we no longer use the AU, or Astronomical Unit; commonly, the light year is used.
Do stars ever land on Earth? ›It is entirely possible for a shooting star to land on the surface of the Earth and it happens quite often. Meteoroids begin to evaporate as soon as they enter the atmosphere, and most of them disintegrate completely several kilometers above the Earth's surface.
Are stars moving or are we? ›The stars seem so fixed that ancient sky-gazers mentally connected the stars into figures (constellations) that we can still make out today. But in reality, the stars are constantly moving. They are just so far away that the naked eye cannot detect their movement.
What is the closest star beyond the solar system? ›Distance Information
Proxima Centauri, the closest star to our own, is still 40,208,000,000,000 km away.
How many planets are outside our solar system? ›
There are 5,388 known exoplanets, or planets outside the Solar System that orbit a star, as of June 1, 2023; only a small fraction of these are located in the vicinity of the Solar System. Within 10 parsecs (32.6 light-years), there are 97 exoplanets listed as confirmed by the NASA Exoplanet Archive.
What is the closest star system with habitable planet? ›Proxima Centauri is known to host one planet for sure—the roughly Earth-size Proxima b, which completes one orbit every 11 Earth days. That puts Proxima b in the star's “habitable zone,” the just-right range of orbital distances where liquid water could exist on a world's surface.
Has there ever been a time where all the planets align? ›Because of the orientation and tilt of their orbits, the eight major planets of the Solar System can never come into perfect alignment. The last time they appeared even in the same part of the sky was over 1,000 years ago, in the year AD 949, and they won't manage it again until 6 May 2492.
What is beyond galaxy? ›Beyond the Galaxy traces our journey from an ancient, Earth-centered Universe all the way to our modern, 21st century understanding of the cosmos.
What is outside of galaxies? ›The space between stars is known as interstellar space, and so the space between galaxies is called intergalactic space. These are the vast empty spaces that sit between galaxies.
Is there a new planet that looks like Earth? ›NASA recently announced the discovery of a new, Earth-sized planet in the habitable zone of a nearby star called TOI-700. We are two of the astronomers who led the discovery of this planet, called TOI-700 e.
Is the Earth A exoplanet? ›Answer and Explanation: No earth is not an exoplanet but a terrestrial planet.
How many galaxies are in the universe? ›It is estimated that there are roughly 200 billion galaxies (2×1011) in the observable universe. Most galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3,000 to 300,000 light years) and are separated by distances on the order of millions of parsecs (or megaparsecs).
How many planets are in the universe? ›If you like big numbers, the exact number is around 10,000,000,000,000,000,000,000,000! All of these planets in the universe orbit around different stars and make up their own solar systems and galaxies. Our solar system, where our planet (Earth) belongs, is also home to a few other planets too.
How long would it take to get to the nearest habitable planet? ›At that speed, it would take a spacecraft about 26 million years to reach Kepler-452b from Earth, if it was going in that direction. Comparison of small planets found by Kepler in the habitable zone of their host stars.
How many habitable planets can orbit a star? ›
For stars with 0.7 or so times the Sun's mass, 6-planet systems do pretty well, too. For some narrow mass ranges of stars, 7-planet systems wind up being stable as well.
What planets could have been habitable? ›| Object | Star | Mass (M⊕) |
|---|---|---|
| Kepler-442b | Kepler-442 | ~2.36 |
| Kepler-62f | Kepler-62 | ~2.8 |
| TRAPPIST-1f | TRAPPIST-1 | 1.04 |
| Teegarden's Star c | Teegarden's Star | ≥1.11 |
The last time five planets were truly in alignment was in the summer of 2022, and that group of planets— Mercury, Venus, Mars, Jupiter, and Saturn—is not expected to be in alignment again until 2040.
What year will all 8 planets align? ›When was the last time all nine planets aligned? In 1982, all nine of the planets were on the same side of the Sun, scattered over some 90 degrees. According to Star Walk, the next full alignment will occur on May 19, 2161, when eight planets will be located within 69 degrees from each other.
What happens when all 9 planets align? ›If all the planets were to align perfectly with each one as close as possible to the Earth, their gravity would raise the ocean tides by just one twenty-fifth of one millimeter. Typical ocean tides on Earth caused by the Moon and Sun are thousands of times larger than that.