Discover the past, present and future of the most important star in our sky
Science | Gregory Beatty | June 10, 2021
There are an estimated 100 to 400 billion stars in the Milky Way Galaxy. In that sweeping, incomprehensible context, our own star — the Sun — is nothing special. But for us, it’s everything: warmth, light and life itself. Sun-based religions and summer/winter solstice celebrations date back millennia, so even the ancients knew that truth.
The folklore is fascinating but the science is neat, too. The story starts 4.5 billion years ago, says University of Regina astronomer Samantha Lawler.
“It starts with these giant clouds of gas,” she says. “Once you get enough gas in a small enough area, it can start to gravitationally collapse. As the gas falls inward, it ends up forming a disk because of angular momentum.”
What Lawler just described is the process by which our solar system formed over millions of years — with a central sphere large enough to generate light and other electromagnetic energy through nuclear fusion, and gas and dust further out clumping together to form the eight planets, dozens of moons and minor planets, and billions of asteroids and comets.
With the summer solstice — the longest day of the year — just around the corner on Sunday, June 20, I spoke to Lawler about the past, present and future of the Sun.
In The Sweet Spot
We know from decades of exoplanet research that stars with planetary systems are quite common. That would seem to make the odds of life existing elsewhere in the galaxy pretty good.
But when it comes to supporting life, not all stars are created equal.
The first point in the Sun’s favour, says Lawler, is that it’s in a relatively remote arm of our spiral galaxy in what astronomers call the Galactic Habitable Zone.
“All of this comes with a caveat, since we only have one example of a planet with life on it,” says Lawler, referring to Earth. “But as you go further out in the galaxy, the density of gas decreases. At some point there’s not enough material to form stars and planets, especially heavy atoms needed for rocky planets.
“As you go further into the galaxy, there are so many stars that close fly-bys could gravitationally tear apart planetary systems,” she says. “Things like merging neutron stars, black holes and supernovae happen more often too — all those big, energetic events that are likely very bad for life. That’s where the idea of the Galactic Habitable Zone comes from.”
Star size is an important factor too. In more sexist times, astronomers used the memory trick “Oh Be A Fine Girl and Kiss Me” for the descending order of stellar classes.
“The Sun is a G-class star so it’s average size,” says Lawler. “There are many smaller, cooler stars, and not many hot, giant stars. The really massive stars (class O and B which shine bluish-white) don’t live very long. They burn through their fuel in a few million years, and we’re not even sure if that’s long enough to form planets, let alone life.”
Fossil records show that life emerged on Earth within a few hundred million years of the Sun and planets forming 4.5 billion years ago but it took another 2.5 billion years for advanced life to start to evolve. The Sun’s projected lifespan is 10 billion years, so we had that luxury of time.
Class K and M stars, which shine reddish-orange, are even longer-lived but they present a different problem, says Lawler.
“They have big flares, which makes them unstable,” she says. “But they seem very good at forming planets. The stars are cooler, so to be in the habitable zone the planets have to be close. But are the flares too much, so they radiate the surface? We don’t know yet. It seems the Sun is a nice balance: it lives for a long time and is fairly stable.”
The Sun stands out a bit too for being a solo star.
“Looking at the stars around the Sun, we know that about half have a companion star or are in multiple star systems,” says Lawler. “There can be incredibly complicated orbital configurations for stars, and we have found planets around many of them. It doesn’t make it impossible for planets to form, although we don’t know about habitability.”
Good News, Bad News
According to astrobiologists, life requires three essentials to emerge: liquid water, carbon and energy. Stars are an obvious source of energy — although possibly not the only one, as even on Earth there are places (such as deep sea vents) where life thrives, fuelled by chemical energy.
But most life on Earth is powered by photosynthesis, with plants using solar energy to grow and reproduce, supporting an integrated food web of herbivores, carnivores, insects, fungi and more.
As much as we’re dependent on the Sun’s energy for survival, as soon as we’re outside Earth’s protective magnetic field that same energy becomes a deadly threat.
And that, says Lawler, presents a big obstacle to space travel.
“You hear talk about ‘Oh, it’s okay if climate change destroys Earth. We can just move to Mars.’ But if you look at the worst-case projections for what Earth’s climate could do, it’s still going to be so much easier to live here than on Mars,” she says.
One big challenge is that Mars, unlike Earth, doesn’t have a magnetic field.
“You’re going to have problems with high energy particles from the Sun, along with cosmic rays which are high energy particles from the rest of the universe,” says Lawler.
Even in low-Earth orbit, astronauts are protected by the magnetic field. Only the 27 astronauts who journeyed to the Moon on Apollo 8 and Apollo 10–17 have experienced that lethal stellar environment — on short missions lasting six to 12 days.
Until we can devise effective ways to shield ourselves from solar and cosmic rays, we’ll have to do most of our deep space exploration with probes. [see sidebar]
In recent years, two probes launched in 1977 (Voyager 1 and 2) have actually transmitted from the solar system’s outer boundary — known as the heliopause.
“Just like the Sun puts out high energy particles in the solar wind, high energy particles also flow through the galaxy,” says Lawler. “Eventually, there’s a point where the Sun’s particles aren’t dense or fast enough to overcome the galactic particles. That’s the heliopause, which is the boundary between the Sun’s influence and the galaxy’s influence.”
When Voyager 1 and 2 breached the heliopause, they were about 122 A.U. from the Sun. Earth is one astronomical unit from the Sun, so that’s really, really far.
But the solar system doesn’t end there, says Lawler.
“The Oort Cloud, which consists of small icy bodies that we sometimes see as comets, extends to between 10,000 and 100,000 A.U.,” she says. “We know it exists because we see comets coming into the inner solar system, and we can measure their orbits so we know how far out they’ll go again. While the Voyager probes have passed out of the Sun’s magnetic and particle influence, they are still in its gravitational influence.”
The Oort Cloud formed out of the same disk of dust and gas as the Asteroid Belt (at 2.8 A.U.) and Kuiper Belt (30 to 100 A.U.).
“But with the Oort Cloud, these are weakly-bound gravitational objects, so they are affected by tides from the rest of the galaxy that tilt their orbits all around the solar system,” says Lawler. “The Oort Cloud is spherical, and we know that because comets come into the Sun from all different angles.”
Just as the Sun created and sustained us, it will ultimately destroy us — or, at least, Earth.
You’ve perhaps heard the bogus anti-climate change claim that the Sun is warming up and that’s causing global heating. Technically, that’s true: the Sun is warming up. But it’s happening over billions of years, so it’s not relevant to climate impacts from a few centuries of fossil fuel use.
“The Sun has been gradually getting brighter, but over billions of years,” says Lawler. “Right now, Earth is on the inner edge of the Sun’s habitable zone. And that will slowly move outward. As that happens, Earth will become like Venus with a runaway greenhouse effect.”
Early in its history, astronomers speculate, Venus was more Earth-like than Earth was back then. But as the Sun warmed up, its oceans began to boil off, unleashing a cascade of vicious feedback loops that turned Venus into a hellscape with a 465 degree C surface temperature and crushing carbon dioxide atmosphere 93 times heavier than Earth’s.
It’s a grisly fate to contemplate for our beautiful planet.
Fortunately it won’t happen for another two billion years.
“In about five billion years, the Sun will expand into a red giant,” says Lawler. “We’re not sure exactly how big it will get. It could expand all the way to Earth’s orbit. It definitely will expand past Mercury and Venus. Then those outer layers will slowly float away, and the only remnant will be a white dwarf. Over billions of years, it will gradually cool.”
The Milky Way Galaxy will continue to exist, of course — not to mention the rest of the universe, with its estimated two trillion galaxies. So again, in that context, the Sun’s death will be no big deal.
But for us…
White Hot Probe Action
While Mars grabs most of the space exploration headlines with no less than 11 probes studying the Red Planet (seven in orbit, four on the ground), the Sun is getting some probe love too. NASA was first with Parker Solar Probe launched in 2018, followed by the European Space Agency’s Solar Orbiter in 2020.
Through a few earlier probes and plenty of observation from Earth (most recently by the Daniel Inouye Solar Telescope which began operating in Hawaii in late 2019), we already know a fair bit about the Sun. Although it must be said, at the stellar level, numbers can get a little surreal. What does an equatorial radius of 695,700 km and mass of 1.9885 x 1030 kg even mean? Well, it means the Sun is 109 times larger than Earth and 330,000 times more massive.
Three-quarters of the Sun’s mass is hydrogen, and the rest is mostly helium. Pressure in the interior is so great that hydrogen atoms fuse together — creating helium, and a shit-ton of energy. Surface temperature is 5,800 degrees Kelvin, while the core reaches 15.7 million K.
The Sun has interior convection currents that churn up plasma; magnetic flux on the surface that spawns sunspots and flares; and a magnificent corona (visible during solar eclipses) where temperatures, for some curious reason, are far hotter than the surface, hitting one million K.
Over the new few years, Parker Solar Probe (PSP) and Solar Orbiter will provide us with our closest look yet at our home star. In fact, scientific papers are already being written using their data.
Réka Winslow is co-author on a paper examining coronal mass ejections that The Astrophysical Journal just accepted for publication. She’s an astronomer at University of New Hampshire. In an e-mail exchange, she said observations made by PSP have provided a wealth of new information on the environment near the Sun.
“Due to its close approaches to the Sun, PSP allows us to sample coronal mass ejections while they are pristine, before they interact with the solar wind and become more complex,” Winslow says.
As the name implies, CMEs are large ejections of plasma from the Sun’s corona. CMEs energize the solar wind, which can damage satellites and disrupt electrical grids and radio communications on Earth, so it’s an important area of study.
Over the next four years, PSP will move progressively closer to the Sun, using repeated loops back out to Venus for a gravity assist. Many of the CMEs that it’s observing, says Winslow, are also being studied by other spacecraft at different distances.
“Such multi-vantage point studies allow us to investigate how CMEs evolve as they move out from the Sun to Earth and the other planets,” she says. “By studying their evolution, we get better at predicting their arrival time and likelihood of causing a geomagnetic storm on Earth.”