March 29, 2023 10:29 pm

On a clear, moonless evening, you may be in a position to see thousands of stars sparkling like jewels above. But a keen eye will notice that they do not all appear alike. Some glow brighter than other individuals, and some show warm red hues.

Astronomers have identified a number of distinctive kinds of stars in the universe, as diverse as tiny brown dwarfs and red supergiants. Stars in the prime of their lives, recognized as principal sequence stars, are normally classified by how hot they are. Given that most star temperatures can not be straight measured, explains Natalie Gosnell, an assistant professor in physics at Colorado College, astronomers have to have to appear at an additional signal: temperature. This is largely inferred by the colour of the light a star emits, which is reflected in lots of names offered to star kinds.  

Every category, nevertheless, is connected. A star moves via several designations all through its lifetime, an evolution shaped by its original mass and the reactions that take place inside the roiling stellar physique. 

In the beginning…

All stars type from a cloud of dust and gas when turbulence pushes sufficient of that material with each other, pressed into 1 physique by gravity. As that clump collapses in on itself, it begins to spin. The material in the middle heats up, forming a dense core recognized as a protostar. Gravity draws even a lot more material toward the creating star as it spins, generating it larger and larger. Some of that stuff may possibly ultimately type planets, asteroids, and comets in orbit about the new star.

The stellar physique remains in the protostar phase as lengthy as material nevertheless collapses inward and the object can develop. This course of action can take hundreds of thousands of years.

The quantity of mass that is gathered through that stellar formation course of action determines the ultimate trajectory of the star’s life—and what kinds of stars it will turn out to be all through its existence.

Protostars, infant stars—and failures

As a protostar amasses a lot more and a lot more gas and dust, its spinning core gets hotter and hotter. As soon as it accumulates sufficient mass and reaches millions of degrees, nuclear fusion starts in the core. A star is born.

For this to take place, a protostar has to accumulate a lot more than .08 instances the mass of our sun. Something much less and there will not be sufficient gravitational stress on the protostar to trigger nuclear fusion. 

These failed stars are named brown dwarfs, and they stay in that state for their lifetime, progressively cooling down devoid of nuclear fusion to assistance release new power. In spite of their name, brown dwarfs can be orange, red, or black due to their cool temperatures. They have a tendency to be slightly bigger than Jupiter, but are a lot a lot more dense.

Protostars that do obtain sufficient mass to turn out to be a star occasionally go via an interim phase. Throughout a roughly ten million-year period, these young stars collapse beneath the stress of gravity, which heats up their cores and sets off nuclear fusion. 

In this stage, a star can fall into two categories: If it has a mass two instances that of our sun, it is deemed a T Tauri star. If it has two to eight solar masses, it is a Herbig Ae/Be star. The most huge stars skip this early stage, due to the fact they contract as well immediately. 

As soon as a sufficiently huge star starts to undergo nuclear fusion, a balancing act starts. Gravity nevertheless exerts an inward force on the newborn star, but nuclear fusion releases outward power. For as lengthy as these forces balance every other out, the star exists in its principal sequence stage. 

The most frequent stars in the galaxy are red dwarfs, such as the 1 illustrated right here blasting a nearby planet with hot gas. NASA, ESA, and D. Player (STScI)

Fueling principal sequence stars

Key sequence stars, which can final for millions to billions of years, are the vast majority of stars in the universe—and what we can see unaided on dark, clear nights. These stars burn hydrogen gas as fuel for nuclear fusion. Beneath the super-hot circumstances in the core of a star, colliding hydrogen fuses, producing power. This course of action produces the chemical components for a reaction that tends to make helium. 

Mass dictates what form of star an object will be through the principal sequence stage. The a lot more mass a star has, the stronger the force of gravity pushing inward on the core and hence the hotter the star gets. With a lot more heat, there is more rapidly fusion and that generates a lot more outward stress against the inward gravitational force. That tends to make the star seem brighter, larger, hotter, and bluer.

[Related: The Milky Way’s oldest star is a white-hot pyre of dead planets]

Several principal sequence stars are also usually referred to as “dwarf” stars. They can variety considerably in luminosity, colour, and size, from a tenth to 200 instances the sun’s mass. The largest stars are blue stars, and they are especially hot and vibrant. In the middle are yellow stars, which includes our sun. Somewhat smaller sized are orange stars, and the smallest, coolest stars are red stars. 

The hottest stars are O stars, with surface temperatures more than 25,000 Kelvin. Then there are B stars (ten,000 to 25,000K), A stars (7,500 to ten,000K), F stars (six,000 to 7,500K), G stars (five,000 to six,000K—our sun, with a surface temperature about five,800K is 1 of these), K stars (three,500 to five,000K), and M stars (much less than three,500K). 

Upsetting the balance to develop a giant star

As a star runs out of fuel, its core contracts and heats up even a lot more. This tends to make the remaining hydrogen fuse even more rapidly: It produces added power, which radiates outward and pushes a lot more forcefully against the inward force of gravity, causing the outer layers of the star to expand.

As these layers spread out, they cool down, and that tends to make the star seem redder. The outcome is either a red giant or a red supergiant, based on if it is a low mass star (much less than eight solar masses) or a higher mass star (higher than eight solar masses). This phase normally lasts up to about a billion years.

Appearing a lot more orange than red, some red giants are visible to the naked eye, such as Gamma Crucis in the southern constellation Crux (aka the Southern Cross).

The vibrant blue star on the correct of this image is Epsilon Crucis, a K-form star in the constellation Crux. NASA/JPL-Caltech/UCLA

The death and afterlife of a low-mass star

Stars die in remarkably distinctive approaches, based on their masses. For a low-mass star, as soon as all the hydrogen is practically gone, the core contracts even a lot more, finding even hotter. It becomes so scorching that the star can even fuse helium—which, due to the fact it is an element heavier than hydrogen, needs a lot more heat and stress for nuclear fusion. 

As a red giant burns via its helium, generating carbon and other components, it becomes unstable and starts to pulsate. Its outer layers are ejected and blow away into the interstellar medium. Ultimately, when all of these layers have been shed, all that remains is the tiny, hot, dense core. That bare remnant is named a white dwarf.

[Related: Wiggly space waves show neutron stars on the edge of becoming black holes]

About the size of Earth, even though hundreds of thousands of instances a lot more huge, a white dwarf no longer produces new heat of its personal. It steadily cools more than billions of years, emitting light that seems anyplace from blue white to red. These dense stellar remnants are as well dim to see with a naked eye, but some are visible with a telescope in the southern constellation Musca. Van Maanen’s star, in the northern constellation Pisces, is also a white dwarf. 

The explosive stellar death of a higher-mass star

Stars with mass eight instances that of our sun normally adhere to a comparable pattern, at least in the starting of this phase. As the star runs low on helium, it contracts and heats up, which permits it to convert the resulting carbon into oxygen. That course of action repeats itself with the oxygen, converting it to neon, then the neon into silicon, and ultimately into iron. When no fuel remains for this fusion sequence, and power is no longer becoming released outward from these reactions, the inward force of gravity immediately wins. 

Inside a second, the outer layers of the star collapse inward. The core collapses and then rebounds, sending a shock wave via the rest of the star: a supernova. 

Life soon after a supernova for a star requires 1 of two paths. If the star had among eight and 20 instances the sun’s mass through its principal sequence stage, it will leave behind a superdense core named a neutron star. Neutron stars are even smaller sized in diameter than white dwarfs, at about the size of New York City’s length, and include a lot more mass than our sun.

But for the most huge stars, that remnant core will continue collapsing beneath the stress of its personal gravity. The outcome is a black hole, which can be as tiny as an atom but include the mass of a supermassive star.

Not all stars match into neat categories

The progression from protostar to white dwarf, neutron star, or black hole may appear simple. But, Gosnell says, a closer appear can yield surprises. The European Space Agency’s Worldwide Astrometric Interferometer for Astrophysics mission, which is making a detailed 3D map of all our galaxy’s stars, has been revealing lots of of these oddball suns. 

A single such instance is a star in a binary or multi-star technique that accretes mass from a companion. With all that added mass to burn, it can appear younger than its accurate age, appearing bluer and brighter. That, Gosnell says, is named a blue straggler star, due to the fact it appears to be “straggling behind its anticipated evolution.” 

Yet another odd form of star is sub-subgiant, Gosnell says. These stars also are located in binary systems, and are transitioning from the principal sequence to the red giant branch, even though they keep dimmer. This sort of subgiant star has “really active magnetic fields with lots of star spots on the surface,” she says. “And so you have these truly magnetically active, visually dynamic stars as the star spots rotate in and out of view.” 

The ongoing ESA mission, she adds, is reviewing stars with a “much finer-toothed comb”—which may possibly reveal the accurate wide variety and complexity of stars that have existed all along. As such missions “peel back the layers,” Gosnell says, “We begin to see truly intriguing stories come out that challenge the edges of these categories.”

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