Herschel helps solve mystery of cosmic dust origins

Supernovae are the answer to what supplied our early universe with dust.

By Jet Propulsion Laboratory, Pasadena, California — Published: July 8, 2011

This layout compares two pictures of a supernova remnant called SN 1987A — the left image was taken by the Herschel Space Observatory, and the right is an enlarged view of the circled region at left, taken with NASA's Hubble Space Telescope. ESA/NASA-JPL/UCL/STScI

New observations from the infrared Herschel Space Observatory reveal that an exploding star expelled the equivalent of between 160,000 and 230,000 Earth masses of fresh dust. This enormous quantity suggests that exploding stars, called supernovae, are the answer to the long-standing puzzle of what supplied our early universe with dust.

"This discovery illustrates the power of tackling a problem in astronomy with different wavelengths of light," said Paul Goldsmith from NASA's Jet Propulsion Laboratory in Pasadena, California. "Herschel's eye for longer-wavelength infrared light has given us new tools for addressing a profound cosmic mystery."

Cosmic dust is made of various elements, such as carbon, oxygen, iron, and other atoms heavier than hydrogen and helium. It is the stuff of which planets and people are made, and it is essential for star formation. Stars like our Sun churn out flecks of dust as they age, spawning new generations of stars and their orbiting planets.

Astronomers have for decades wondered how dust was made in our early universe. Back then, Sun-like stars had not been around long enough to produce the enormous amounts of dust observed in distant, early galaxies. Supernovae, on the other hand, are the explosions of massive stars that do not live long.

The new Herschel observations are the best evidence yet those supernovae are, in fact, the dust-making machines of the early cosmos.

"The Earth on which we stand is made almost entirely of material created inside a star," said Margaret Meixner from the Space Telescope Science Institute in Baltimore, Maryland. "Now we have a direct measurement of how supernovae enrich space with the elements that condense into the dust that is needed for stars, planets, and life."

The study focused on the remains of the most recent supernova to be witnessed with the naked eye from Earth. Called SN 1987A, this remnant is the result of a stellar blast that occurred 170,000 light-years away and was seen on Earth in 1987. As the star blew up, it brightened in the night sky and then slowly faded over the following months. Because astronomers are able to witness the phases of this star's death over time, SN 1987A is one of the most extensively studied objects in the sky.

Initially, astronomers weren't sure if the Herschel telescope could even see this supernova remnant. Herschel detects the longest infrared wavelengths, which means it can see cold objects that emit little heat, such as dust. But it so happened that SN 1987A was imaged during a Herschel survey of the object's host galaxy — a small neighboring galaxy called the Large Magellanic Cloud.

After the scientists retrieved the images from space, they were surprised to see that SN 1987A was aglow with light. Careful calculations revealed that the glow was coming from enormous clouds of dust, consisting of 10,000 times more material than previous estimates. The dust is about –429° to –416° Fahrenheit (–256° to –249° Celsius) — colder than Pluto, which is about –400° Fahrenheit (204° Celsius).

"Our Herschel discovery of dust in SN 1987A can make a significant understanding in the dust in the Large Magellanic Cloud," said Mikako Matsuura from University College London in England. "In addition to the puzzle of how dust is made in the early universe, these results give us new clues to mysteries about how the Large Magellanic Cloud and even our own Milky Way became so dusty."

Previous studies had turned up some evidence that supernovae are capable of producing dust. For example, NASA's Spitzer Space Telescope, which detects shorter infrared wavelengths than Herschel, found 10,000 Earth masses worth of fresh dust around the supernova remnant called Cassiopeia A. Hershel could see even colder material, and thus the coldest reservoirs of dust. "The discovery of up to 230,000 Earths worth of dust around SN 1987A is the best evidence yet that these monstrous blasts are, indeed, mighty dust makers," said Eli Dwek, from NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

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FRED BAER from ARIZONA said:

Try this web page for a table of stellar lifetimes: http://c2h2.ifa.hawaii.edu/Tutorial/pages/star_lifetime.htm

This web page has a calculator that lets you enter the number of solar masses and get a lifetime:
http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/startime.html

Some things to keep in mind:

The first web page states that metallicity plays a very small role, compared to stellar mass, in stellar lifetimes. However, a text I have shows that a very low metallicity can considerably reduce lifetimes, with the lower the mass the greater the difference.

These calculations seem to be for the time spent on the main sequence, just turning hydrogen into helium in a star's core. You get a somewhat higher lifetime if you include subsequent stages of fusion.

These calculations are based on various mathematical models of stellar evolution. The models might be good enough to give fairly accurate answers, but I doubt there is any way to determine how well the models reflect reality.

About the the source of helium on earth - There is little helium in the atmosphere, because it tends to float off into space. Because helium is an inert gas, you don't find helium in chemical compounds. Helium is generally (or maybe always) captured from oil wells. I gather there was some debate as to how much of the helium was captured when the earth was formed versus how much comes from radioactive decay. (Some radioactive decay involves the emission of alpha particles which are just helium nuclei). I believe it is now accepted that most helium is the result of radioactive decay.

Submitted: 7/27/2011 4:15:24 AM (CST)

JAMES D SMITH from WASHINGTON said:

Stephen, I think the Hydrogen and Helium, on Earth, were created at the beginning of the universe, and not in Supernovae.
On a related note, does anyone know of a table of correlation, between the mass of a star, and its' lifetime? I.E., how long does a 20 Sol mass star live, as opposed to a 50 Sol mass?

Submitted: 7/19/2011 8:40:05 PM (CST)

STEPHEN ARMSTRONG from CALIFORNIA said:

Wow! I had no idea that exploding stars were called "supernovae"! And I thought that Earth was ENTIRELY made up of material from an ancient supernova. I wonder what percentage of Earth ISN"T made up of the stuff from that supernova, and how Ms. Meixner arrived at that conclusion.

Submitted: 7/15/2011 8:00:47 PM (CST)

CHRIS SCHUR from ARIZONA said:

It seems often the smallest and most insignificant seeming images have the greatest impact on our understanding of the universe. The tiny blip in the Herschel image shows the creation of the type of matter that makes all things on earth possible. It reminds me of the tiny barely visible blip in the lunar prospector images, where that barely visible fluff of light coming from the center of the dark area inside Cabeus crater signified the release of huge amounts of lunar water! What will the tiny details reveal on the close ups of Vesta we are bout to see in coming weeks? ;)

Submitted: 7/8/2011 11:55:30 AM (CST)

	Mercury	Mercury	Venus	Venus
RISE	—		—
SET	—		—
		—		—

Herschel helps solve mystery of cosmic dust origins

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