The Soell system was the site of a galactic disaster. The ancient Forerunners fought a long war against intelligent parasites called the Flood. As a last resort, the Forerunners built a ring-shaped superweapon orbiting the moon of Soell’s largest planet. Triggering the weapon, the Halo Array, wiped out the Flood, the Forerunners and all other intelligent life in the galaxy. For millennia, the star Soell was forgotten — until humans found the Halo.
The popular video game Halo and its fictional stars were Julián Alvarado Gómez’s obsession 15 years ago. As a young man in Bogotá, Colombia, he played Halo and its offshoots competitively. Today, at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., he’s studying an actual star, Iota Horologii.
His goal is to map the star’s magnetic field over time along with its gusting stellar wind, the stream of energetic particles that defines a star’s territory and batters its planets. This work will help him understand what our star was like in its youth and how it influenced the start of life on Earth.
“One of the big difficulties we have in our understanding of the sun is that we only have one sun,” says the 35-year-old astrophysicist. Getting to know another star that has a similar mass and temperature as the sun, referred to as a “sunlike” star, would shore up astronomers’ grasp of the sun. And it would offer details on how sunlike stars may affect potential life on their orbiting planets.
Alvarado Gómez’s road to this stellar career was rocky. In 2003, he was about a third of the way through an undergraduate physics degree at the National University of Colombia in Bogotá when financial trouble put his studies on hold. He filled his time with friends and Halo. “We were practicing a lot,” he jokes.
The game offered a solution to his money woes when he came in third place in a Halo tournament. His friend Julián Hernández took first. The two caught the attention of Microsoft representatives looking for skilled players to help advertise the game by playing in public.
From there, he and Hernández earned a salary for playing at events, plus the occasional bonus for winning a tournament. It was fun, Alvarado Gómez says, and it paid the bills.
After two years, he returned to school to study physics again. “That was a very hard semester,” he says. After an early focus on the sun, he shifted toward the stars for his Ph.D. at the European Southern Observatory in Garching, Germany.
That shift has brought Alvarado Gómez to some surprising differences between the sun and other sunlike stars. His research has also revealed how a star might protect its planets from its own energetic outbursts. Yet the work didn’t take him as far from Soell as he’d thought.
The best way to get to know a star is through its magnetic field, Alvarado Gómez says. The sun is the most familiar example: Our star’s temperamental behavior and periodic mood swings are thought to exist thanks to changes in magnetism.
Magnetic fields help heat the sun’s wispy outer atmosphere, the corona, to millions of degrees Celsius. Those magnetic fields also help drive a stream of charged particles out into space (SN Online: 8/11/17). That solar wind blows a bubble that defines the boundary of the solar system (SN Online: 12/10/18). It can also batter unprotected planets; scientists think the solar wind stripped away much of Mars’ atmosphere.
When tangled magnetic field lines on the solar surface suddenly snap, powerful eruptions of plasma called coronal mass ejections break free (SN: 4/13/19, p. 15). When strong CMEs hit Earth, they can fry satellites, shut down power grids and damage living cells.
The sun’s magnetic activity waxes and wanes in about an 11-year cycle. The peak, or maximum, rages with sunspots, CMEs and bright radiation flashes called flares, while the minimum is relatively quiet.
In another magnetic quirk, the direction of the sun’s dominant magnetic field flips at the peak of each cycle (SN: 3/3/01, p. 139). As the sun’s inner engine reorganizes itself, the south magnetic pole switches to the north, and vice versa. This polarity reversal has ripple effects on the solar wind that extend to the edges of the solar system.
Other stars share much of this magnetic fickleness. About 60 percent of sunlike stars show signs of magnetic cycles of varying lengths depending on the stars’ ages. Young sunlike stars of a few hundred million years have shorter cycles and emit more flares than the 4.6-billion-year-old sun.
“You can use other stars to show snapshots of the sun at earlier and later periods in its evolution,” says stellar physicist Travis Metcalfe of the Space Science Institute in Boulder, Colo. “What was the sun like in the past? What will it be like in the future?”
From what we can tell, stars seem to tame their magnetic frenzies as they age. They calm down by losing mass through their stellar winds and CMEs (SN: 8/31/19, p. 11).
But young stars can be rough on their planets. Stinging winds and violent outbursts could wipe out life as efficiently as the Halo Array, unless something stopped or blocked them (SN Online: 3/5/18).
The perfect star
To find out when a star’s planets face the most danger, Alvarado Gómez needed a magnetic view of a young sun, from the field on its surface to the edges of its stellar wind.
Finding one turned out to be a massive undertaking. The perfect star had to be similar to the sun in mass and temperature, two features that determine a star’s life span. And it needed an observable magnetic cycle in its corona. “If [stars] have corona, they have stellar winds,” Alvarado Gómez says.
That final requirement was the trickiest. Astrophysicists have measured the cycles of fewer than 100 stars, based on variations in a particular wavelength of near-ultraviolet light that can be seen from ground-based telescopes. Tracking these cycles takes time, but is relatively easy.
The coronas of stars other than the sun, however, are best observed through the high-energy X-rays they emit. “I wanted a star in which I could be confident that I know what the activity cycle looks like,” Alvarado Gómez says. “The best proxy for that — the best of all — is the X-rays.” But the only way to see these X-rays is from space, a much more difficult and expensive prospect.
Very few stars have had their magnetic cycles recorded in X-rays. When Alvarado Gómez was searching for his target star in 2014, there were only four solar mass stars that would work, and three of them were in orbits with another star. Alvarado Gómez ruled those out, fearing the companion stars could mess things up. “There was only one star left,” he says. “Iota Horologii.”
Our star’s younger self
The sun and Iota Horologii have enough in common to make the distant star a good stand-in for a young sun.
|Surface temperature||5,500° Celsius||5,800° Celsius|
|Radius||695,500 km||806,780 km |
(1.16 solar radii)
|Mass||2 x 1030 kg||2.46 x 1030 kg |
(1.23 solar masses)
|Rotation period||24 days||8.2 days|
|Magnetic activity cycle||11 years||1.6 years|
|Age||4.6 billion years||625 million years|
|Distance from Earth||150 million km||530 trillion km|
|Planet at Earth orbit||Earth||A gas giant 2.5 times |
Back to the Halo
Iota Horologii, located about 56 light-years (530 trillion kilometers) from Earth, is similar to the sun in temperature, size and mass. At about 625 million years old, it’s the youngest star with a detected magnetic activity cycle.
Its age is more or less the age of the sun when life appeared on Earth, says astrophysicist Jorge Sanz-Forcada of the Center for Astrobiology in Madrid. “This is a way to observe how the sun was at the moment when life appeared.” The star has a planet, too. Unfortunately, it’s an uninhabitable gas giant, but its orbit lasts almost a full Earth year: 307 days.
Even better for observers, Iota Horologii has the shortest magnetic activity cycle observed to date. It peaks and falls over just 1.6 years, Sanz-Forcada, Metcalfe and astrophysicist Beate Stelzer of Eberhard Karls University in Tübingen, Germany, reported in 2013 in Astronomy & Astrophysics. Researchers could observe the star’s full cycle almost seven times in the time it takes the sun to cycle once.
To go after Iota Horologii, Alvarado Gómez got access to every telescope he could, stockpiling more data than astronomers usually get for a single star. “This became a much bigger project than what was envisioned,” he says. “We wanted to map the magnetic cycle. But then we realized that there’s much more that you can do.”
Between October 2015 and September 2018, he and colleagues observed the star using the High Accuracy Radial velocity Planet Searcher, or HARPS, spectrograph at La Silla Observatory in Chile. Then he teamed up with Sanz-Forcada’s group to watch the star in X-rays, ultraviolet and visible light using a trio of space telescopes.
He also gathered another 13 years of data from previous observations of Iota Horologii in a wavelength of light that tracks magnetism on stars’ surfaces. “We were able to trace it back all the way to 2002 — when I was playing Halo,” Alvarado Gómez says. He now has data on more cycles for Iota Horologii than astronomers have for the sun in certain wavelengths.
Astronomers have watched Iota Horologii’s 1.6-year magnetic cycle repeat several times. Observations span more than six years in near-ultraviolet light, polarized ultraviolet light and X-rays, all ways to measure magnetic activity. The orange square at the top right might be a stellar flare.
Iota Horologii’s short magnetic cycle
“These types of observations are rare, especially for new stars that haven’t been observed a lot in the past,” Metcalfe says. “It’s enormously helpful to our understanding of where cycles come from and where they’re going.”
In late 2017, when Alvarado Gómez was writing the first paper on the HARPS observations, he learned Iota Horologii had a potential Halo connection that blew him away.
While searching for information about the star, he stumbled on a fan site laying out the case that the Halo star Soell is supposed to be Iota Horologii. The two have matching planets and similar properties and positions in the sky. It’s possible, says Frank O’Connor, Halo franchise creative director at 343 Industries in Redmond, Wash. “Our normal process includes referencing our sci-fi against current scientific consensus, understanding and data. So it almost certainly got checked against real star systems … and may indeed be the same one,” he says.
“I just find it amazing,” Alvarado Gómez says. He recalled his graduate adviser, astronomer Gaitee Hussain at ESO, telling him that stellar physicists fall a little bit in love with the objects they study. “I was already in that process with Iota Horologii,” he says. He took the potential connection to the game that helped get him back to school as “a sign that I should keep working on it.”
Far beyond the sun
For all that Iota Horologii resembles the sun, its magnetic life looks subtly different in important ways, Alvarado Gómez and colleagues found. Those differences could hold clues to how sunlike stars change over time, and whether those changes influence their planets.
For one thing, Iota Horologii’s magnetic field flips like the sun’s — but faster. The sun flips once every cycle, so it takes two cycles to return to its original configuration. Iota Horologii’s cycle is 1.6 years, but its polarity flips every 1.2 years. That speedy somersault could suggest that the internal engine that drives a star’s magnetic field is different in young stars than in older ones.
Iota Horologii’s magnetic activity cycle is also surprisingly stable, according to a paper the team posted September 3 at arXiv.org that will also appear in Astronomy & Astrophysics. Four cycles in a row lasted the same amount of time and reached the same activity levels.
“We never expected so much regularity,” Sanz-Forcada says. “In the sun, [the cycle] is not so regular.”
The sun’s highest activity level varies from one cycle to the next; the most recent solar cycle had one of the wimpiest peaks ever recorded (SN: 11/2/13, p. 22). No one is sure why. But if Iota Horologii represents the sun in its youth, then the sun’s cycles may have been more consistent a few billion years ago.
Alvarado Gómez is working on figuring out what all the data mean for Iota Horologii’s stellar wind — and by extension, what winds could do to planets. He’s making the first maps of the strength and direction of Iota Horologii’s entire magnetic field at every point of the star’s surface. He’ll then use the maps to build computer simulations of the shape and strength of Iota Horologii’s stellar wind.
The structure of the sun’s winds (top row) changes with magnetic activity, as shown in these simulations. At low activity, the faster wind (green) swirls at the sun’s poles, as slower wind (magenta) centers around the equator. At high activity, the wind grows more complex. Iota Horologii’s winds are more variable throughout its magnetic cycle, data and simulations show.
He’s also trying to observe the edge of Iota Horologii’s stellar wind directly by looking for the star’s hydrogen wall, a sheet of ultraviolet light produced when a star’s wind slams into atoms from the surrounding interstellar environment (SN: 9/15/18, p. 10).
The hydrogen wall marks the edge of the star’s sphere of influence. Measuring the wall’s properties could reveal how much of Iota Horologii’s mass is being carried away by the winds, and whether that mass loss changes with the stellar cycle.
Unfortunately, Iota Horologii showed no sign of a wall when Alvarado Gómez searched for one with the Hubble Space Telescope in September 2018. But he thinks he saw a wall for an even younger sunlike star called HD 147513, located about 42 light-years from Earth. If that’s confirmed, the finding will be a major step toward learning how young stars lose mass, calm down and stop pummeling their planets.
Coronal mass ejections can also fling mass from stars and fry nearby planets. “If we know very little about winds, we know even less about CMEs,” Alvarado Gómez says. Cracking the CME code could solve another stellar mystery: How do these violent young stars, which emit flares much more often than the sun, avoid quickly burning themselves out? And are their planets safe?
On the sun, the rare solar flares, sudden bright flashes of high-energy light, are almost always accompanied by CMEs. The brighter the flare, the bigger and faster the CME. Not so for young sunlike stars.
Some young stars emit bright flares nonstop — but without monstrous CMEs. Only one confirmed CME has been caught fleeing a star other than the sun in real time (SN Online: 8/7/18). Alvarado Gómez’s collaborator Sofia-Paraskevi Moschou, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, identified only 12 more possible CMEs in a historical review in the May Astrophysical Journal.
That may be just as well. If those stars really were burping out enormous CMEs as often as they emit huge flares, Moschou says, “it would strip away all the energy of the star after a few events.”
Alvarado Gómez thinks he knows what keeps the CMEs contained: a magnetic cage. This effect has been seen on the sun. The largest sunspot observed in the past 30 years, called AR 2192, sprouted in 2014. This spot generated hundreds of flares, some of which were in the strongest category ever observed. But none of those flares included a CME. The strong magnetic fields in that sunspot may have arced across the region and acted as a magnetic cage, preventing any CMEs from escaping.
Alvarado Gómez and colleagues think that in younger stars, the cage envelops the entire star, not just one star spot, the team reported in 2018 in the Astrophysical Journal.
With a cage like that, “you could stop almost all the CMEs we have ever observed in the sun,” he says. Even the CMEs that manage to escape would be slower and less energetic than expected from observed flares.
That’s exactly what Moschou found: Bright flares give off slower CMEs than expected. That could be good news for the habitability of planets near these stars, although Alvarado Gómez thinks the case may be more mixed.
“The bad news is that this energy has to go somewhere,” he says. It could go back into the star to power more flares, which also could be bad for life on orbiting planets (SN Online: 3/5/18).
Like young stars that slow their cycles over time, Alvarado Gómez has slowed his gaming. “I still play, but not on the same kind of level,” says Alvarado Gómez, who will move to the Leibniz Institute for Astrophysics Potsdam in Germany this fall.
But he’s reminded of his youthful obsession each time he opens his laptop to work on a stellar simulation. Halo’s main character, Master Chief, is the image staring out from his screen.