Astronomy Thought-provokers

The universe is filled with interesting objects that tell some of the most fascinating stories in astronomy. Today, among other topics, we heard about weird x-ray objects and the possibility of searching out civilizations in globular clusters. If anybody in Hollywood needs a writer to cover those ideas for the next galaxy-spanning Sci-Fi epic, I’m your person!

The Andromeda Galaxy’s X-ray Binaries

So, what happens when an object sends out lots of x-ray radiation? Obviously, it catches the attention of astronomers because it’s very likely telling us that something fascinating is happening. X-rays are generated in very hot, active environments, such as near black holes or in the explosions of massive stars. Our Sun also gives off x-rays, particularly as it generates strong outbursts called flares.

Our galaxy — and many others — has a collection of objects called x-ray binaries. Each one contains a black hole or a neutron star accompanied by a stellar companion. These pairs give off prodigious amounts of x-rays.

Astronomers looked at the nearest spiral galaxy to us — called the Andromeda Galaxy — and 40 of its x-ray binaries. They used the Nuclear Spectroscopic Telescope Array, or NuSTAR, to study them and figure out what role they played in early galaxy formation. Some researchers suspect that these objects could have been responsible for heating up intergalactic gas clouds in the early universe. The idea is to study these x-ray binaries in a “nearby” galaxy and then apply what we learn about them to similar objects that existed when the universe was very young.

In x-ray binaries, material from the companion star can “spill over” and then be captured by the strong gravitational pull of the nearby black hole or neutron star. That material gets heated to incredibly high temperatures, and the result is an intense release of x-rays.

NuSTAR looked at a swath of Andromeda, which revealed the 40 binaries. Now astronomers are working on identifying which ones may have black holes and which are powered by neutron stars. These remnants of stellar evolution and their companion stars may have played a very central part in heating the early universe.

Life in a Crowded Stellar Suburb

As you probably know, astronomers have been finding planets around other stars for a few decades now. The big question — do they bear life? — is the question that is, so far, unanswered. At least one of those planets has been found in a globular cluster, which is a collection of a million (or more) stars packed into an area of space about 100 light-years across. That led astronomer Rosanne DiStefano of Harvard-Smithsonian Center for Astrophysics to wonder whether more planets might exist in globular clusters. And, since she was engaging in “what if” scenarios, she speculated about what civilizations could exist on those planets.

“A globular cluster might be the first place in which intelligent life is identified in our galaxy,” she said today.

Globular Clusters? Really?

At first glance, globular clusters don’t seem like great places to grow planets. They’re old — they formed about 10 billion years ago. Their stars contain fewer of the heavy elements needed to make planets. Those elements (like iron and silicon) are created in earlier generations of stars. Some scientists have argued that this makes globular cluster stars less likely to host planets. However, DiStefano and Alak Ray (Tata Institute of Fundamental Research, Mumbai) think that view is much too negative.  “It’s premature to say there are no planets in globular clusters,” Ray said.

If you look at a globular cluster, it seems like it would be too crowded to allow planets to exist in stable orbits around their stars. However, it turns out that if a star’s habitable zone is close enough in, a planet could exist in a reasonably stable orbit. And, if it’s an Earth-like planet around an older, cooler star, that zone would be pretty close.

Here’s another thing about those clusters. Since they are old, their most massive stars have died off, leaving behind the older, cooler stars — and a supply of heavier elements that could be useful in planet formation.

So, it does seem possible that habitable planets can form in globular clusters and survive for billions of years. Take the next step and endow them with civilizations, and you have a whole new set of thoughts to consider about the possibilities for life on them, and what globular cluster civilizations would be like.

Such a civilization would enjoy a very different environment than our own. The nearest stellar neighbors would be just a trillion miles away, That would make interstellar communication and exploration quite easy, compared to what we have to do here on Earth to talk to the neighbors.

“We call it the ‘globular cluster opportunity,'” said DiStefano. “Sending a broadcast between the stars wouldn’t take any longer than a letter from the U.S. to Europe in the 18th century.”

“Interstellar travel would take less time too. The Voyager probes are 100 billion miles from Earth, or one-tenth as far as it would take to reach the closest star if we lived in a globular cluster. That means sending an interstellar probe is something a civilization at our technological level could do in a globular cluster,” she said.

The closest globular cluster to Earth is several thousand light-years away, making it difficult to find planets from our vantage point. It’s a big problem in the core of a cluster. That’s because the stars there are really jammed in together. But, it could be possible to detect planets on the outskirts of globular clusters. Astronomers might even spot free-floating planets by using applications of gravitational lensing. That occurs when the planet’s gravity magnifies light from a background star.

A more intriguing idea might be to target globular clusters with SETI search methods, looking for radio or laser broadcasts. That concept has a long history: the late astronomer Frank Drake used the Arecibo radio telescope in 1974 to broadcast the first deliberate message from Earth to the globular cluster Messier 13 (M13).

Okay, so we don’t KNOW if there are planets and civilizations in globulars, yet. But, DiStafano and her colleagues have raised some cool things to think about as we look for exoplanets in our galaxy — and beyond. THAT is one of the great attractions of astronomy!