Gravity has been making waves - literally. Earlier this month, the Nobel Prize in Physics was awarded for the first direct detection of gravitational waves two years ago. But astronomers just announced another huge advance in the field of gravitational waves - for the first time, we’ve observed light and gravitational waves from the same source.
There was a pair of orbiting neutron stars in a galaxy (called NGC 4993). Neutron stars are the crushed leftover cores of massive stars (stars more than 8 times the mass of our sun) that long ago exploded as supernovas. There are many such pairs of binaries in this galaxy, and in all the galaxies we can see, but something special was about to happen to this particular pair.
Each time these neutron stars orbited, they would lose a teeny bit of gravitational energy to gravitational waves. Gravitational waves are disturbances in space-time - the very fabric of the universe - that travel at the speed of light. The waves are emitted by any mass that is changing speed or direction, like this pair of orbiting neutron stars. However, the gravitational waves are very faint unless the neutron stars are very close and orbiting around each other very fast.
As luck would have it, the teeny energy loss caused the two neutron stars to get a teeny bit closer to each other and orbit a teeny bit faster. After hundreds of millions of years, all those teeny bits added up, and the neutron stars were *very* close. So close that … BOOM! … they collided. And we witnessed it on Earth on August 17, 2017.
Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet
LIGO is a ground-based detector waiting for gravitational waves to pass through its facilities on Earth. When it is active, it can detect them from almost anywhere in space.
The other thing that happened was what we call a gamma-ray burst. When they get very close, the neutron stars break apart and create a spectacular, but short, explosion. For a couple of seconds, our Fermi Gamma-ray Telescope saw gamma-rays from that explosion. Fermi’s Gamma-ray Burst Monitor is one of our eyes on the sky, looking out for such bursts of gamma-rays that scientists want to catch as soon as they’re happening.
And those gamma-rays came just 1.7 seconds after the gravitational wave signal. The galaxy this occurred in is 130 million light-years away, so the light and gravitational waves were traveling for 130 million years before we detected them.
After that initial burst of gamma-rays, the debris from the explosion continued to glow, fading as it expanded outward. Our Swift, Hubble, Chandra and Spitzer telescopes, along with a number of ground-based observers, were poised to look at this afterglow from the explosion in ultraviolet, optical, X-ray and infrared light. Such coordination between satellites is something that we’ve been doing with our international partners for decades, so we catch events like this one as quickly as possible and in as many wavelengths as possible.
Astronomers have thought that neutron star mergers were the cause of one type of gamma-ray burst - a short gamma-ray burst, like the one they observed on August 17. It wasn’t until we could combine the data from our satellites with the information from LIGO/Virgo that we could confirm this directly.
This event begins a new chapter in astronomy. For centuries, light was the only way we could learn about our universe. Now, we’ve opened up a whole new window into the study of neutron stars and black holes. This means we can see things we could not detect before.
The first LIGO detection was of a pair of merging black holes. Mergers like that may be happening as often as once a month across the universe, but they do not produce much light because there’s little to nothing left around the black hole to emit light. In that case, gravitational waves were the only way to detect the merger.
Image Credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)
The neutron star merger, though, has plenty of material to emit light. By combining different kinds of light with gravitational waves, we are learning how matter behaves in the most extreme environments. We are learning more about how the gravitational wave information fits with what we already know from light - and in the process we’re solving some long-standing mysteries!
NGC 2841 is a striking spiral galaxy located in Ursa Major, teaming with cosmic dust & gases dancing around its center. Older, yellow stars reside in the galaxy’s heart, whereas the younger, blue stars live along the twisting spirals. It was discovered by William Herschel in the late 1780′s. (from the book Hubble’s Universe: Greatest Discoveries and Latest Images)
What’s happening to this spiral galaxy? Just a few hundred million years ago, NGC 2936, the upper of the two large galaxies shown, was likely a normal spiral galaxy – spinning, creating stars – and minding its own business. But then it got too close to the massive elliptical galaxy NGC 2937 below and took a dive.
Dubbed the Porpoise Galaxy for its iconic shape, NGC 2936 is not only being deflected but also being distorted by the close gravitational interaction. A burst of young blue stars forms the nose of the porpoise toward the right of the upper galaxy, while the center of the spiral appears as an eye. Alternatively, the galaxy pair, together known as Arp 142, look to some like a penguin protecting an egg. Either way, intricate dark dust lanes and bright blue star streams trail the troubled galaxy to the lower right.
In a billion years or so the two galaxies will likely merge into one larger galaxy.
Closest Supernova In Years Brings Cosmic Fireworks To Earth’s Skies
“Cosmic fireworks like these don’t truly happen at random; they are clustered in time and space around the most massive, intense star-forming regions of all. You can’t have a bigger star-forming region than one that includes the entire galaxy, and the sweeping, grand, irregular arms of the Fireworks galaxy are as good as they come. Based on what we see, we expect this elevated rate to continue for more than a million years.”
Every once in a while, a new light appears somewhere in the night sky: the result of a massive star reaching the end of its life. From many millions of light years away, the brilliance of a supernova shines across the cosmos. Just a few days ago, a new light was discovered in a galaxy only 22 million light years away, making it the closest supernova discovered in three years. The galaxy housing it is a hotbed of supernova formation, having been home to ten such explosions in the past 100 years: more than we’ve found in any other galaxy. The reason? This entire galaxy, despite having only half the stars of the Milky Way, is a giant star-forming region. Starburst galaxies like this are the best place to look for cataclysmic events like this, and NGC 6946 is maybe the best example of all.
Comet 41P/Tuttle-Giacobini-Kresak poses for a Messier moment in this telescopic snapshot from March 21, 2017. In fact it shares the 1 degree wide field-of-view with two well-known entries in the 18th century comet-hunting astronomer’s famous catalog. Sweeping through northern springtime skies just below the Big Dipper, the faint greenish comet was about 75 light-seconds from our fair planet.
Dusty, edge-on spiral galaxy Messier 108 (bottom center) is more like 45 million light-years away. At upper right, the planetary nebula with an aging but intensely hot central star, the owlish Messier 97 is only about 12 thousand light-years distant though, still well within our own Milky Way galaxy.
Named for its discoverer and re-discoverers, this faint periodic comet was first sighted in 1858 and not again until 1907 and 1951. Matching orbit calculations indicated that the same comet had been observed at widely separated times. The comet 41P orbits the Sun with a period of about 5.4 years.