Key findings about how many suns there are in the Milky Way
- How many suns there are in the Milky Way depends on your definition of sun. According to astronomers, our Sun is considered a main sequence G star.
- To find out how many suns there are in the Milky Way, astronomers measure the light emitted by the galaxy. From these measurements, they estimated that no more than 10 percent of the stars in the galaxy are similar to our Sun.
- Using the same measurement techniques, astronomers will likely be able to predict the number of suns in the universe. The number is so large that it is difficult to understand. Carl Sagan once compared the number of stars in the universe to the number of grains of sand on all the beaches on Earth and again found that about 10 percent or less of them would be like our Sun.
The Milky Way galaxy alone contains billions of stars, although as it branches out from our corner of the universe, NASA estimates The cosmos itself may contain up to one septillion stars, followed by 24 zeros. Among them, one of them is our Sun.
However, how many other suns could inhabit this vast fabric of space depends on how strict your definition is.
“You often hear in textbooks that our sun is like a typical average star,” says Christopher Palma, a professor of astronomy and astrophysics at Pennsylvania State University. “It is in the middle of the range of masses and temperatures. Many of the properties of the Sun are clearly average.”
This makes our Sun a main sequence G star, burning hydrogen at its core. It is near the center of the scale that runs from O, B, A, F, G, K and M. By this criterion alone, the easiest way to answer this question is to ask how many main sequence G stars there are.
Read more: 10 Facts You May Not Know About the Milky Way
How many suns are there in the Milky Way?
One way astronomers find out how many stars there are in the Milky Way is by measuring light.
“For example, we can often get really accurate estimates of the number of molecules in a glass of water. But in astronomy we work on detecting light from objects that are very far away, and that's all we need,” says Palma. “We see a flash of light and we try to figure out how much is in that flash of light.”
Palma compares it to the light of a single LED and a flashlight containing thousands of them: a flashlight would be a thousand times brighter than the light of a single LED. To extrapolate this to galaxies, astronomers work backwards.
“A typical galaxy like the Milky Way emits that much light. If a typical star is so bright, how many stars add up to the same amount of light emitted by the Milky Way?” Palma continues.
Such estimates are easier to make by looking at distant galaxies, including Andromeda.
“We live about halfway from the center of the galaxy, and of course we can't see the whole galaxy because we're in it,” says Kevin France, assistant professor of astrophysical and planetary sciences at the University of Colorado. “If you're in a cloud, you can't see the shape of the cloud because you can only see what's right around you.”
Ultimately, based on the light emitted by the Milky Way, our galaxy contains hundreds of billions of stars, with a lower limit of about 100 billion—and among them, Palma adds, a few to ten percent are expected to be similar to our Sun.
How many suns are there in space?
Astronomers can similarly estimate the number of stars in the entire universe. If the Milky Way is a typical galaxy, France says, astronomers could first estimate how many other Milky Ways inhabit our cosmic neighborhood.
“What is the volume of this little corner of the universe that we see compared to what we think is the volume of the entire universe?” France continues. “It's something similar. Basically, you can multiply that number and get the total number of stars.”
This amount can be difficult to comprehend: a number of zeros that complete a number that few people have ever encountered in their lives. Astronomer Carl Sagan compared the total number of stars in the Universe to the number of grains of sand on all the beaches on Earth. Likewise, about ten percent of them may be stars like the Sun.
“Although you'll never count them all individually, you can point a telescope at a patch of sky and ask, 'How many galaxies do I see in this patch of sky?' – says Palma. “Then you can do the math and extrapolate it to the entire universe.”
Read more: To understand the Universe, scientists study the Milky Way's cosmic neighbors
Which planet has more suns?
One might wonder how many of the billions of sun-like stars float adrift and alone in space, and how many are instead tied to planets. Stars can exist in twin systems or even triplets, although an extra planet in the mix will not have a stable enough orbit to remain in the system.
Meanwhile, our solar system consists of eight planets attached to a single star, a configuration that is more common than multiple stars associated with a single planet.
“Usually one star has several planets. We have found examples of other systems with seven planets. We have eight,” says Palma. “But we can't see them all right now.”
As for how many of these stars belong to planets that could actually support life, that's even harder to figure out. The reason they're so hard to detect is that the stars literally outshine the planets, “by a billion times,” according to Palma. If our Sun is the bright eye of a lighthouse, then the Earth can be thought of as the light from the tail of a firefly.
So one can challenge current estimates of the number of sun-like stars by asking how many are sun-like because they support life on planets. This is where the simple answer gets complicated: from G stars to Earth 2.0.
Moreover, astronomers are asking the opposite question: Does a planet like Earth have to have a sun star to be like Earth?
“That’s the million-dollar question,” France says.
Stars like the Sun and planets like the Earth.
How Earth-like planets arise is a question France answered in a recent study published in Astronomical magazine.
The magnetic fields of stars twist, twist and collapse as they rotate, generating enormous amounts of radiation. As they slow down and age over time, the level and intensity of radiation also changes. Too much high-energy radiation for too long can destroy a planet's atmosphere, making it inhospitable.
“This means that the planets orbiting that star are also undergoing the same changes. If you have a really short period of time where you're exposed to a lot of high-energy radiation, maybe your atmosphere will survive,” France says.
Understanding the relationship between a host star and its planet is key to determining what makes a second Earth.
“You can't separate a planet from its star,” France says. “They are connected.”
upcoming NASA Habitable Worlds Observatoryof which France is a part, aims to one day launch an instrument into space that can block the blinding light of stars and catch fireflies on satellite planets. More precisely, France adds, that's about 10 billion blocked stellar photons for every planetary photon.
Once the observatory has captured enough nearby stars, the counting begins: which of those Sun-like stars have a planet in the ideal location, and which of those planets have an atmospheric imprint that could indicate biology.
The technology is still evolving, but it's bringing scientists closer to answering one of humanity's oldest million-dollar questions: Are we alone?
“We live in a very fortunate time where we can technically begin to answer this question,” France says.
Read more: JWST may have discovered a Milky Way twin
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