Observational Astronomer Alberto Bolatto Answers Questions About Galaxy and Star Formation
The College of Computer, Mathematical, and Natural Sciences hosted a Reddit Ask-Me-Anything spotlighting research on how galaxies and stars came to be.
University of Maryland Astronomy Professor Alberto Bolatto participated in an Ask-Me-Anything (AMA) user-led discussion on Reddit to answer questions about galaxy and star formation on February 20, 2026.
Bolatto’s research uses state-of-the-art observational tools—including radio and infrared data from the Atacama Large Millimeter/submillimeter Array (ALMA) and the James Webb Space Telescope—to unveil the mysteries of star formation.
As co-investigator on the PRobe Far-Infrared Mission for Astrophysics (PRIMA), Bolatto is working to help reveal nascent stellar systems with greater precision than ever before. If our probe proposal is funded, the PRIMA team will analyze protoplanetary disks—collections of gas and dust orbiting young stars that are the birthplace of planets—to determine how much water is needed for different types of planets to form.
This Reddit AMA has been edited for length and clarity.
I read that the Milky Way is around 60 galactic years old. How will it develop after thousands of galactic years? Will it still have the arms, or should the shape change?
While it is true that a galactic year is not a standard unit, 60 sounds about right. Indeed, we expect the Milky Way to interact with Andromeda, the other major member of the Local Group, in a few billion years. Over longer periods of time, the major change would be that stars will stop being formed as the gas from which they form is consumed. So our galaxy will become a "red and dead" galaxy with only red low-mass stars shining and remnants of planets and stars. As to how the shape will change, it depends on the details of the interaction, but the Milky Way will likely lose its arms and become more like an elliptical galaxy.
What is the current thinking and data on initial planetary formation and then subsequent migration and collision within a solar system before it settles down?
Our current best theories and supporting observations still tell us that planets form in disks orbiting young stars and may experience migration and collisions as the gas clears and the debris settles. Some of our best data comes from radio interferometers like ALMA, which provides its data to the public (an example is here), and much more recently, JWST (an image is here).
How far away can we see planetary discs?
We can get great detailed imaging when the planetary disks are a few hundred parsecs away, but it is possible to detect them (or their infrared excess emission) much farther away.
How, if at all, have accepted models of early Galaxy formation changed since Webb began operational?
Certainly, the Webb Space Telescope is having a big impact. It is still too early to tell its full impact, but we have found several mysterious classes of new objects, such as the Little Red Dots (LRDs). LRDs are compact red sources, some of which exist at very early times in the universe and could be massive black holes that are growing very fast, or very compact clusters of young stars, or something completely different. These were the objects that caused people to think there were massive galaxies very early in the history of the universe.
Is every spiral galaxy destined to eventually become a globular-elliptical shape as they age? Are there any examples of "young" elliptical galaxies?
Not necessarily. A galaxy becoming more spherical mostly depends on its interactions with other galaxies, which are more common in dense environments like galaxy clusters, or if you let a long time go by.
There are examples of rejuvenated elliptical galaxies, which happen when they merge or swallow a smaller gas-rich galaxy. They start forming stars again because of the injection of gas. Here is an example of a rejuvenated elliptical galaxy, if you'd like to take a look.
How did the theorized super-massive black holes at the center of most galaxies originally form? How old are they? And how do these fit into the broader scale of clusters/superclusters?
We have pretty good proof of the existence of supermassive black holes at the centers of several galaxies. We think they form through the merging of lower-mass black holes; although they also grow by swallowing gas and nearby stars. However, it is unclear what the original seeds are that these black holes grow from. Some measurements suggest that they need to be much more massive than black holes that we think are the result of stellar evolution. The original seeds exist in the very early universe, since we observed black holes at very large cosmological distances.
Is it possible to map dark matter into areas of higher and lower density? Do we even have a detector capable of doing this? And how do theories currently explain galaxies separating so that their dark matter no longer is in the region where visible matter in the form of stars exists?
Yes, we can map dark matter, but usually on much larger scales—not on the scale of the structure of the Milky Way. The usual technique to do this involves gravitational lensing, and it's based on studying how the shape of background galaxies is affected by the gravity of the intervening dark matter as light is bent by it. Here is a recent map of the structure of dark matter on the scale of galaxies and galaxy clusters.
Dark matter is not collisional, which means that it doesn't behave like a gas. It doesn't experience pressure, only gravity. That makes it respond differently from gas to interactions between galaxies and separate from normal (baryonic) matter.
How do the interstellar conditions differ within spiral arms versus between spiral arms versus "outside" the galaxy? And what is the definition of being outside a galaxy?
The density and pressure of interstellar gas are different, as is the radiation field it sees. The interstellar gas is denser in the spiral arms and more tenuous in between the arms. Outside a galaxy, the gas is even lower density and hotter, and usually fully ionized.
There are many definitions of being outside a galaxy, depending on what you are talking about. Maybe for this, we would talk about a gas being outside a galaxy if the radiation field it sees is mostly the intergalactic radiation field. But we make a distinction between circumgalactic gas and intergalactic gas, depending on how close to a galaxy the gas is and where it came from.
Do astronomers even need to visit Earth-based telescopes anymore or can it all be done remotely?
Some telescopes require people to operate them in person, but increasingly, the largest facilities (such as ALMA) have professional operators who send the data to observers. Some other facilities can require the astronomer to attend observations remotely, such as the Keck or Green Bank telescopes.
Heavy elements exist on Earth but are only created in stars. How did they get here? Was everything in our solar system previously in a different star that exploded? If so, how was there enough hydrogen left to create yet another star?
Yes, pretty much everything around us that is not hydrogen or helium came from a star that exploded. There is plenty of hydrogen around that was never processed by stars and comes from the Big Bang, and this hydrogen coalesces into new stars. Most of the helium also has a primordial origin and is not the result of stellar processing. The material that formed the solar system experienced at least 2 or 3 generations of enrichment, cycling through being in a star that exploded as a supernova.
What do you think is the most surprising or counterintuitive insight your research has revealed about how galaxies evolve, especially when comparing the Milky Way to others, and how might upcoming missions like PRIMA reshape our understanding of the role water plays in planet formation?
Galaxies as massive as the Milky Way are relatively uncommon. Most galaxies are much smaller. Something that has surprised me is the realization that the Milky Way is made up of little remnants of other galaxies that have been swallowed or merged over cosmic time; the Milky Way is not a single entity, but the result of many entities merging together. It didn't always exist this way; it accumulated over time.
Another thing that may be counterintuitive is that galaxies not only gain mass, but they also lose a lot of mass—for example, ejecting gas from their central regions. So they are very dynamic systems, where things are happening all the time. Here is a JWST picture from my own work of a nearby galaxy that is very actively forming stars (known as a starburst) and ejecting a lot of gas and dust from its central regions.
We think water in the form of ice is key to forming large Jupiter-like planets. This is because it is more efficient to grow mass from hydrogen in the form of water ice than from hydrogen in the form of gas. PRIMA can test these theories of planet formation because it is able to observe spectral lines from water at low temperatures and ices. So it can help establish where and how much of this water exists in protoplanetary disks.
What more can you tell us about the Little Red Dots?
We have many competing theories as to what these Little Red Dots are. They were a complete surprise! There could be some exotic origins, but I think the current data favor that these objects are an extreme version of what we call "active galactic nuclei" in the local universe, that is, massive black holes that are accreting and growing, but enshrouded in a lot of gas and dust.
