If There’s One, There are Probably Others
I didn’t get to write about this yesterday, what with all the other exciting astronomy results being thrown at us this week. But, there’s a big, massive, bright, dying star out there that’s going to blow up. It’s called Eta Carinae, visible from Earth’s surface from the Southern Hemisphere. It’s so fascinating, astronomers want to know if there are any others like it out there. So, they looked through data from Spitzer Space Telescope and Hubble Space Telescope to see if any are hidden away in other galaxies.
Eta Carinae is actually two stars in a 5.5-year orbit. The larger, brighter one (the primary) is a luminous blue variable that started out its life with at least 150 times the mass of the Sun. Over time, it has lost the equivalent of about 30 Suns. That’s the monster star which will blow sometime soon. By that, I mean, it will explode as a class of supernova so huge and powerful and bright that astronomers call it a “hypernova”.
Once it does blow up, essentially “any time now”, Eta Carinae will appear incredibly bright in our sky. It would be much brighter than Venus, but it’s not likely we’d suffer any ill effects from it, due to distance.
Eta Carinae lies some 7,500 light-years away and it suffered an eruption in the 1840s that caught everyone’s attention. At that time, it blew off about ten Sun’s worth of material off at that time. What we see now as a sort of “dumbbell” shape is that ejecta traveling out to space. Add up the massive star, the expanding material, and the known evolution of this thing, and you’ve got a one-of-a-kind object that continues to fascinate astronomers.
Are There More of them Out There?
In astronomy and science in general, we don’t like to see just “one” of anything. While our Sun is special to us, there are more stars like it. The more sunlike stars we study, the more we understand them (and our star) as a “type” and where they fit into the pantheon of all possible stars. It’s the same with Eta Carinae. It’s fascinating, but if there’s one, there should be others. And, they can tell us more about the type of stars Eta Carinaes are.
It turns out there are others out there. A study using archival data from Spitzer and Hubble found five similar objects in other galaxies. And that’s good.
Why Look for more Members of the Eta Carinae Family?
As one of the nearest laboratories for studying high-mass stars, Eta Carinae has been a unique, important astronomical touchstone since 1840s activity. To understand why that eruption occurred and how it relates to the evolution of massive stars, astronomers needed additional examples. Catching rare stars during the short-lived aftermath of a major outburst approaches needle-in-a-haystack levels of difficulty, and nothing matching Eta Carinae had been found prior to the discovery of others in the Hubble and Spitzer data.
Finding Eta Carinae Twins
Dust forms in gas ejected by a massive star. This dust dims the star’s ultraviolet and visible light, but it absorbs and re-radiates this energy as heat at longer, mid-infrared wavelengths. Those “signals” show up in Spitzer data, and could be matched with objects seen in Hubble observations. But, first astronomers had to find candidate objects.
An initial survey of seven galaxies from 2012 to 2014 didn’t turn up any Eta twins, which meant they must be rare, indeed. It did, however, identify a class of less-massive and less-luminous stars of scientific interest, demonstrating the search was sensitive enough to find objects like Eta Carinae, had they been present.
In a follow-on survey in 2015, astronomers found two candidate Eta twins in the galaxy M83, located 15 million light-years away. There were also likely Eta Carinaes lurking in NGC 6946, M101, and M51, located between 18 million and 26 million light-years away. These five objects mimic the optical and infrared properties of our galaxy’s Eta Carinae, indicating that each very likely contains a high-mass star buried in five to ten solar masses of gas and dust.
While astronomers study those new candidates more fully, our own Eta Carinae continues to tick along, approaching its hypernova stage. It’s going to be magnificent when it goes.
One question I get from people is, “Has it already blown up?”
That’s entirely possible, and we just haven’t seen it yet. After all, it IS 7,500 light-years away, so if it has already died, there’s a bright surprise waiting us in the future. You can read more about this finding at the Hubble Space Telescope Web site.