Postdoc Spotlight: Alexander Ji

What questions are you pursuing in your career?

The questions that drive me are about the first stars and galaxies--those that formed in the first billion years of the universe. Right after the Big Bang the universe is kind of boring, but then about 150 million years afterwards, the first stars begin turning on. These stars completely change the universe around them: Blowing giant gas bubbles, synthesizing the first heavy elements (things like carbon, oxygen, iron, gold, and silver), forming the first black holes, and seeding the formation of the first galaxies. I like studying these early stars and galaxies because they’re our oldest ancestors. We still have almost no observations in this early epoch, which means we’re still trying to answer the most basic questions. What types of stars were there in the early universe? What types of elements did they make? How fast did the first galaxies grow, and how many were there? 

How is your research done?

I am a near field cosmologist, which is a lot like being an archaeologist. Archaeologists dig up old artifacts to learn about the history of ancient humans. We look for objects as old as the universe to learn about the first stars and galaxies. Astronomers have a unique advantage over other historical sciences: since the light from distant stars takes a very long time to get to us, we get to cheat and “look back in time” in order to study older stars. But the types of objects I’m interested in are too far away to see directly, even with the next generation of telescopes. So instead we study old objects that have survived from the early universe. I mostly study two types of old objects.

The first way is to look for old stars. It is hard to precisely measure stellar ages, so instead we use a proxy which is the total amount of heavy elements that exist inside a star. After the Big Bang, the only elements that existed were hydrogen and helium. As stars form and die they create heavier elements, meaning that stars with very small amounts of heavy elements are likely to have been born in the first billion years of the universe. Our Sun is about 2 percent heavy elements, which makes it kind of young. I am looking for stars that are about 13-14 billion years old and have less than .0002 percent heavy elements.

The second way is to look for old galaxies--ultra-faint dwarf galaxies. These relic galaxies were not good at making stars, only forming stars for about a billion years before being shut off. But this is great for us because they preserve a clear snapshot about what the first galaxies are like. 

What is one of your biggest discoveries?

We discovered how the heaviest elements (like gold, silver, and platinum) are made in the early universe. The origin of these elements had been a mystery since the 1950s, but it was either in supernova explosions or in merging neutron stars. In 2016, we found an old galaxy that was chock-full of these heaviest elements, clearly showing that those elements were made in merging neutron stars. Our discovery foreshadowed another fantastic discovery in 2017, where for the first time LIGO directly detected merging neutron stars in gravitational waves and Carnegie astronomers played a leading role in showing that those merging neutron stars also synthesized gold, silver, and platinum. Between these two discoveries, it’s pretty clear now that merging neutron stars beat out supernovae as the origin of the heaviest elements.

When did you realize you were interested in astronomy and what path did you take to get there?

I knew when I went to college that I wanted to do something related to physics, philosophy, and/or psychology because I wanted to understand as much as possible in the universe. While I studied all of those things, the most fun for me was physics. I realized that the fields that were making the most progress in understanding the biggest questions were astronomy and astrophysics. However, I was still unsure that I wanted to pursue astronomy when I went to graduate school. I received a lot of different advice on how to progress to graduate school, but the best advice was if you don’t know the right direction, find a person you’re excited to work with. My advisor at MIT was Anna Frebel, who specializes in the earliest stars, and she inspired me to really focus on astronomy. 

For me, astronomy is fun because it combines all the parts of physics that I really enjoy and it also gives me the opportunity to apply science I learned in a lab to things that are far away and back in time. My work gives me a feeling of deep connection that spans the whole cosmos--in a way, it makes me feel one with the universe. 

What has the Carnegie Fellowship meant to you?

Oh, so much. Carnegie is one of the best places to be a scientist. For me, it’s a combination of the people and the freedom. 

This is one of the top-notch best places for anyone, but in particular for me.  My field would not have existed without the astronomers that are here now: George Preston, Ian Thompson, Steve Shectman, Andrew McWilliam, and Josh Simon. The search for old stars is kind of a Carnegie special. Additionally, I have the opportunity to work with others here. It’s a diverse place in terms of the types of things that people work on and that means I get to connect my work with other branches of astronomy. For example, I’m currently working with [Carnegie Fellow] Allison Strom on the connection between nearby galaxies and the formation of faraway galaxies. The Observatories is large enough that there are people working on all kinds of different things, but small enough that you actually get to meet and talk to all those people.  

The freedom here is also really important because we are allowed to pursue the driving questions that get us out of bed. The thing that keeps me in this field is how fun it is and how many connections there are between different aspects of astronomy. Carnegie really supports that and it’s something I truly appreciate.

And then there’s the access to Magellan, of course. The Observatories is a one-of-a-kind place that gives access to big, important telescopes to relatively young scientists. Carnegie encourages you to go out on a limb and try things that are really hard and I like to think I’ve done that in my time here.