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Professor Alison Sills  
Area: Astrophysics (Theoretical)
Alison Sills
Location: ABB 353
Phone: 905-525-9140 ext 24189
Fax: (905)546-1252
  1. Research Profile
  2. Letter to Grad Students

Alison Sills - Stellar lifecycles

Stellar lifecycles

The study of stellar evolution is as old as astronomy itself. "It's the study of how stars change with time, why they look the way they do at different points in their life, how they're formed, and what drives their evolution," said Alison Sills, associate professor of astrophysics in the Department of Physics and Astronomy at McMaster University.

Much like human beings, stars are born, grow older and die, but their lifespan can take billions of years. A star consists mostly of hydrogen gas, which fuels nuclear fusion at its core. When a star runs out of hydrogen, it collapses and dies.

Blue stragglers are a type of star that doesn't fit the traditional model of stellar evolution. Since they are often found in star clusters, which are groups of stars that formed at approximately the same time, they should have similar properties, such as brightness and temperature. Blue stragglers are the black sheep of the stellar family because they don't share the same properties as their fellow stars. "If you look at them more carefully, they're actually younger than the rest of the stars," Sills explained. "Stars form from gas, but in a cluster, there's no gas. There's absolutely nothing for these stars to form from."

The formation of blue stragglers is different from other stars. They can form through a stellar collision, in which two stars collide and form a new star, or through a binary merger, in which two stars that orbit each other move closer together until they combine. Both processes make younger, more massive stars, but astronomers are still trying to find out which process happens more often in the universe.

Globular star clusters, which are some of the oldest objects in the universe, are teeming with hundreds of thousands of stars. With so many stars moving in the cores of these clusters, collisions and mergers are bound to happen. The orbits of two pairs of binary stars may overlap, causing the stars to collide or switch partners.

Computer modelling enables Sills to test her theories without waiting millions of years to see the results. "If you want to merge two stars, you divide them up into little blobs of gas, stick them in your computer and let them collide with each other and see what happens," she explained. She can also determine what a star is made of by looking at its emission and absorption lines. The position and intensity of the lines indicate the star's composition, temperature and surface pressure.

The supportive learning environment in the Department of Physics and Astronomy ensures that new discoveries are met with congratulations, while requests for assistance never go unanswered. "You get a result, and you can go around and say, ‘Look what I found,' and everyone's excited," said Sills. "At the same time, you can say, ‘I'm really struggling with this, I just don't understand, I can't figure this out,' and people are very happy to offer suggestions."


 December 16, 2015

Alison Sills
Department of Physics & Astronomy
McMaster University

Dear Prospective Graduate Student,

My work is theoretical, and covers stellar evolution, stellar dynamics, and hydrodynamics. I am interested in stellar populations that occur in unusual circumstances, such as stellar collisions in globular clusters. I study both the populations themselves and the information that the populations can give us about the environment they are in. While most of my work involves computer simulations, I also have experience working with optical observational data, particularly from the Hubble Space Telescope.

I have been at McMaster since July 2001. My most recent students worked on observations of globular cluster properties in M87 (with Bill Harris); theoretical models of star cluster formation (with James Wadsley); models of very young star clusters including both the stars and the gas; and on models of globular clusters that were formed in a dwarf galaxy which was then accreted by the Milky Way. I usually take on one or two summer or senior thesis undergraduate students as well. I may have room in my group for an exceptional graduate student this year, and have a variety of projects available, including some involving special-purpose computing hardware for stellar dynamics, binary star modelling, cluster formation and early evolution, and global cluster properties.

Astronomical research is a collaborative process, and I try to treat graduate students as collaborators as much as possible. The first few years of graduate school are an important transition from being an undergraduate student (when the answers to all questions are known) to becoming an independent researcher (when asking the right questions is as important a skill as finding the answers). All my students choose projects based on a combination of their interests and mine. MSc projects are better-defined from the beginning, and tend to involve more input from me, while PhD projects are more often suggested and planned by the students themselves. I encourage students to attend conferences, both in Canada and abroad, and I expect them to publish the results of their research in international journals at all stages.

Please contact me at  if you think this is a research area you might be interested in or if you have any other questions.