Looking for Eta Carinae Twins in Distant Galaxies

If There’s One, There are Probably Others

eta carinae from HST

A huge, billowing pair of gas and dust clouds are captured in this stunning NASA Hubble Space Telescope image of the supermassive star Eta Carinae. Courtesy NASA/ESA/STScI

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.

eta carinae candidates in other galaxies

Researchers found likely Eta twins in four galaxies by comparing the infrared and optical brightness of each candidate source. Infrared images from NASA’s Spitzer Space Telescope revealed the presence of warm dust surrounding the stars. Comparing this information with the brightness of each source at optical and near-infrared wavelengths as measured by Hubble, the team was able to identify candidate Eta Carinae–like objects. Top: 3.6-micron images of candidate Eta twins from Spitzer’s IRAC instrument. Bottom: 800-nanometer images of the same sources from various Hubble instruments.
Courtesy NASA, ESA, and R. Khan (GSFC and ORAU)

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.

Stay tuned!

Attention, Luminous Blue Variable Fans!

More Outburst Data from Eta Carinae

There’s an unstable star out there that’s on the brink of destruction. It lies some 7,500 light-years away, embedded in a star-forming region called the Carina Nebula. The star itself is called Eta Carinae (Eta Car, for short), and it’s actually a double star system. Some 170 years ago, observers noticed that it was growing very bright. Over time, it became the second-brightest star in the sky.

This unexpected brightening came to be known as the “Great Eruption”, and astronomers of the time watched it with great interest — but didn’t have much in the way of sophisticated instrumentation to really dig into the object to tell what was going on there.  Today, modern telescopes are showing us what really happened during the Great Eruption.

The story goes like this:  the more massive member of the duo — a type of star called a luminous blue variable — began blasting out huge amounts of its own mass. Over the period of 20 years that it was seen to be erupting (from 1837 to 1858), this heaving star lost more than 20 solar masses of material.  Much of that “star stuff” can still be seen in a double-lobed cloud surrounding the system.

These images reveal light from a massive stellar outburst in the Carina Nebula reflecting off dust clouds surrounding a behemoth double-star system. The color image at left shows the Carina Nebula, a star-forming region located 7,500 light-years from Earth. The massive double-star system Eta Carinae resides near the top of the image. The star system, about 120 times more massive than the Sun, produced a spectacular outburst that was seen on Earth from 1837 to 1858. But some of the light from the eruption took an indirect path and is just now reaching our planet. The light bounced off dust clouds (the boxed region at the bottom of the image, indicating an area of space that lies about 100 light-years away ) and was rerouted to Earth, a phenomenon called a light echo. The image was taken in February 2000 by the U.S. National Optical Astronomy Observatory's Curtis Schmidt Telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile. The three black-and-white images at right show light from the eruption illuminating dust clouds near the doomed star system as it moves through them. The effect is like shining a flashlight on different regions of a vast cavern. The images were taken over an eight-year span by the U.S. National Optical Astronomy Observatory's Blanco 4-meter telescope at the CTIO. Credit: NASA, NOAO, and A. Rest (Space Telescope Science Institute, Baltimore, Md.)

Eta Car (the LBV) is a massive dying star.  Such stars do not make up the majority of stellar systems in our galaxy, so of course, astronomers are quite interested in what sort of death process Eta Car will go through. Unlike our Sun, which will sort of gently swell to become a red giant (and lose much of its mass in a less-explosive manner), Eta Car will likely go out in a huge cataclysm called a supernova.  Some astronomers suggest it could be such a catastrophic event it would be termed a “hypernova.”

However it blows, Eta Car’s passing will afford astronomers with a ringside seat to stellar mass destruction. The first thing they’ll detect when it blows is a gamma-ray burst that could affect our communications satellites.  After that, they’ll be busy cataloguing the process of the explosion and the ring of debris that will be rushing out to space.  Some astronomers have suggested that this explosion could happen anytime from the next few years to a few million years from now.  Regardless of when it blows, it’s not likely to hurt us much on Earth, since the rotational axis of the system is pointed away from us.

Hubble Space Telescope has been observing Eta Car for a couple of decades now, watching subtle changes in the cloud surrounding the stellar pair, and analyzing the mixes of gas and dust in that cloud. The most recent observations actually pinpoint an echo of the light from the Great Eruption bouncing off more distant parts of the clouds.   The observations of the light echoes mark the first time astronomers have used spectroscopy to analyze a light echo from a star undergoing powerful recurring eruptions, though they have measured this unique phenomenon around exploding stars called supernovae. In spectroscopy, light from an object is captured by the telescope and sent to an instrument (called a spectrograph) that breaks that light into all its wavelengths. Each wavelength of light tells you something about the chemical makeup of the object, its speed through space, whether or not it is spinning, and gives a measure of its temperature.  For the Hubble observations, the spectrograph  captured Eta Car’s characteristic “fingerprints,” providing details about its behavior, including the temperature and speed of the ejected material.

The light echoes from Eta Car are telling astronomers that this restless, heaving old star system does not behave like other stars of its class.  The temperature of the outflow from Eta Carinae’s central region, for example, is about 8,500 degrees Fahrenheit (5,000 Kelvin), which is much cooler than that of other erupting stars. So,this gives some important clues about what’s happening inside the star.  There are other clues in the stream of light HST is studying, and light from the outburst is still on its way to Earth. Astronomers are expecting another brightening in about six months and that will give them more data to chew on as they seek to understand how this star is going through its death process. So, stay tuned.  There’s more news on the way from Eta Car!