Jeremy Webb

Postdoctoral Fellow

University of Toronto

 

Life Inside A Globular Cluster

 

 

For the July 2014 issue of Astronomy Magazine, Professor William E. Harris and I were asked to submit an article describing what life would be like if Earth was orbiting a star that lived within a globular cluster. For the article, we took dynamical simulations of globular clusters and placed our theoretical planet at various locations within the model. Not only were we able to determine what life would be like for astronomers on such a planet, but we used our simulations to generate images of what the night sky would look like for anyone living inside a globular cluster. Our model cluster is 12 Gyr old, contains 570,000 stars, and would orbit above the plane of the disk like clusters found in the Milky Way. We allowed our planet to be orbiting a parent star that had an eccentric orbit bringing it from within the core of the cluster to 4 times the half-light radius of the cluster.

 

More recently, with the help of UITS Research Technologies at Indiana University, we developed a 3D visualization of what the nighy sky would like if you lived on such a planet. The visualization, which can be accessed here, can be viewed in any browser (just use the arrow keys to navigate and your mouse to access the menu). It also has the ability to be used in Virtual Reality mode with a mobile device. This visualization is still a work in progress, so please email me at jerjwebb@iu.edu to report any bugs or ask questions. A detailed description of what life would be like on a planet within a globular cluster, including some additional visualizations of the night sky at various stages of the planet's orbit, can be found below:

 

First we considered when the planet and its parent star are located within the core of the globular cluster, close to its center. At this distance the local density is thousands of times the local stellar density, with the closest stars orbiting around the same distance as the Oort cloud. The night sky, imaged to the left, would contain 130,000 stars visible to the naked eye. The brightest stars in the sky would have apparent magnitudes of -9 (100 times the brightness of Venus).

Being an astronomer on a planet at the center of a globular cluster would be a rather limited occupation. The average sky brightness at night would be twenty times brighter than our own sky during a full moon, making observations rather difficult. While it may seem that stellar astronomers would be quite happy living at the center of a cluster, stellar astronomy would be hindered by the fact that measuring distances to stars within the cluster would be difficult due to the lack of visible fixed background sources. Radio and x-ray telescopes would be able to see beyond the cluster, but it would be difficult interpreting their signals without having an optical companion to compare to.

 

 

Moving outward, we next considered when the planet was found near the half-light radius of the cluster (2.45 pc). At this distance, the brightest object in the sky would be the main body of the cluster. However, far from the cluster center one would be able to faintly see distant globular clusters as well as the bulge and disk of the host galaxy. Doing astronomy from this vantage point is a bit of an improvement over the cluster's core. It's also an improvement over doing astronomy here on Earth, as dimming and reddening effects due to interstellar dust in the Galactic disk would not be an issue. The trade-off would be not being able to study the early stages of stellar evolution, as interstellar clouds of gas and dust that lead to star formation are primarily found in the disk.

 

 

Perhaps the most exciting time to live on such a planet would be when its parent star was at apogalacticon, with the image on the left featuring the night sky when the planet is 10 pc from the cluster center. While you would still get a spectacular view of the cluster when its center was in the night sky, astronomy could easily be performed at locations that faced away from the cluster at night. Essentially, this distance offers the best of both worlds.