Category Archives: star death

The End of Sun-like Stars

Planetary Nebulae

Several times a year I go out and give public talks about astronomy and one of the questions I get a lot is, “What will happen to the Sun?”  Sometimes people have this idea that the Sun will blow up in a huge explosion and overtake Earth. Others worry about something hitting the Sun and causing it to do something.  Actually, things DO hit the Sun— comets do this, for example. But so far, none has made a difference in how the Sun behaves.

This intriguing new picture from ESO’s Very Large Telescope shows the glowing green planetary nebula IC 1295 surrounding a dim and dying star located about 3300 light-years away in the constellation of Scutum (The Shield). This is the most detailed picture of this object ever taken.
This intriguing new picture from ESO’s Very Large Telescope shows the glowing green planetary nebula IC 1295 surrounding a dim and dying star located about 3300 light-years away in the constellation of Scutum (The Shield). This is the most detailed picture of this object ever taken.

What DOES make a difference in how the Sun (and other stars) acts are age and mass. Stars with masses ranging from one solar mass to about 8 solar masses have fairly quiet deaths — that is, they don’t blow up in titanic explosions so much as they just “puff out” their outer atmospheres to space and then fade away.

The Sun is the one we care the most about. It is about 4.6 billion years old and it will likely live another four billion years before it starts to age and die. That aging process is of great interest to astronomers and so they study other stars as they die to see how the Sun will do it. The Sun and stars like it (similar in mass and luminosity) shine for billions of years before they hit retirement age and start to swell up.

As they do this, their atmospheres get “huffed off” by a stellar wind similar to our solar wind. It’s almost as if the star is gently sneezing its outer layers to space. This takes a while — and all that material eventually ends up in a cloud of gas and dust that surrounds the cloud. That cloud (with the dying star at the center) is what’s called a “planetary nebula”. The name was bestowed by William Herschel, who thought they looked similar to a distant gas giant planet.  There’s nothing planetary about these things — they’re really stars like the Sun moving through an important step in the aging and death process. 

Planetary nebulae come in many different shapes.  This image comes from the European Southern Observatory’s Very Large Telescope. It’s of a nebula called IC 1295, and since the image is such high resolution, you can actually make out multiple shells of material surrounding the dying star. This implies the atmosphere blew out in episodes as the star’s faltering core emitted sudden bursts of energy.

The gas surrounding the dying star (which is the small blue-white spot in the heart of the nebula next to a reddish spot) is bathed in strong ultraviolet radiation from the aging star, which makes the gas glow. Different chemical elements glow with different colors, and the green color you see here comes from ionized oxygen (that is, oxygen gas heated by radiation from the central star and is now emitting greenish light).

This cloud won’t last forever. In a few tens of thousands of years, the clouds will slowly dissipate. Eventually only the remains of the star will be left behind as a white dwarf.  It will continue to shrink a bit longer, but eventually that will stop and the white dwarf will continue to cool for billions of years. I read somewhere that in the entire history of the universe, not one white dwarf has yet cooled to completion. There hasn’t been time in the 13.8-billion-year age of the cosmos for them do that.

So, that’s the fate of the Sun in general. It won’t blow up as a supernova (because it doesn’t have the mass to do so). It will gently (for a star) sigh its life away. Hopefully by that time, humanity will have found other worlds to live on.

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!