Exploding galaxies
While the Milky Way hasn’t had a supernova in 400 years, other galaxies are bursting with them
Astronomers all over the world dream of a bright supernova. We haven’t caught such an event in our own Galaxy since before the telescope was invented, and a typical system like the Milky Way might expect no more than one a century. Some galaxies seem blessed; M61, for example, a rare spiral galaxy in the Virgo Cluster, has had eight since 1900.
Why some galaxies should have more supernovae is something of a puzzle.
Most supernovae represent the death of massive stars, and so galaxies with more supernovae are likely to have more massive stars. As such behemoths burn through their fuel quickly, living for only a few hundred million years or so, their presence indicates recent star formation. Supernovae are therefore signs that a galaxy is alive and happily making stars.
Galaxies that have more supernovae do have higher rates of star formation but otherwise appear completely normal, with little to distinguish them from their neighbours. This month’s paper takes a close look at NGC 2770, a spiral galaxy that appears to buck this trend. Three definite supernovae have been found in the galaxy in the last 20 years – in 1999, 2007 and 2008 – and a fourth candidate, in 2015, is either a supernova or an outburst from a type of giant star called a luminous blue variable. Despite these fireworks, NGC 2270 seems to have a normal, rather than an enhanced, star formation rate. What’s going on?
“NGC 2770’s high star-formation rate may be because it has just survived an interaction with a neighbouring galaxy”
Counting stars
Previous attempts to work out how many stars NGC 2770 is forming depended on observations made with a radio telescope in Westerbork, the Netherlands. Hydrogen is the fuel for star formation and so finding a galaxy rich in the gas usually means a more productive stellar nursery. Based on these observations, NGC 2770 seemed to have a star formation rate of about one solar mass per year
The Sky at Night
– more or less the same as our own Milky Way. To explain the observed burst of supernovae, a rate of 20 times that would be necessary.
So, how do we explain this difference? In this month’s paper, a team led by Michal Michalowski of the Adam Mickiewicz University in Poland lay the blame on dust, so often a confounding factor in observations. Dust – particles of silicon and carbon – absorbs light, making the galaxy appear fainter in the wavelengths that reveal star formation than would otherwise be the case. Dust is associated with star formation, so it’s not unexpected for the two to go together, but the size of the correction is surprising. Once dust is properly measured and accounted for, the star formation rate for the galaxy is nearly 50 solar masses per year – more than enough to explain the supernovae. But why does NGC 2770 have such a high star formation rate? One possibility is that it may have just survived an interaction with a neighbouring galaxy; there’s a faint bridge of matter linking the main galaxy to one of its companions, a typical sign of an ongoing merger. Such interactions often stir up gas, triggering star formation. It’s possible that our own Milky Way might undergo a similar burst of star formation in a few billion years when it interacts with Andromeda. Impatient astronomers may not want to wait that long, though, and will be keeping their fingers crossed in the meantime. interaction
Read it online at: https://arxiv.org/abs/2008.08091
ne of the more frustrating things about being an astronomer is that we can’t go and have a close look at our favourite objects. Instead, we have to gather as much information as we can while we’re stuck here on Earth. That means collecting radiation across the electromagnetic spectrum (as well as particles and gravitational waves where we can).
I think of it as resembling a jigsaw – where each bit of information we collect, whether it’s visible light, radio waves or X-rays, is like a separate piece. Only when you put them all together do you get a complete picture.
This year marks the 75th anniversary of the first observations at Jodrell Bank Observatory (originally called the Jodrell Bank Experimental Station). Bernard Lovell established the station after he returned from the Second World War as he was determined to use the radar technology (which he had helped develop for defence) to further his study of cosmic rays – high-speed charged particles arriving from outer space.
In his attempts to detect radar echoes from these cosmic rays – a task he never managed – Lovell built larger and larger radio aerials. These turned out to be perfect for studying radio emissions from the distant Universe. He had accidentally become one of the first radio astronomers.
ORadio revolution
For thousands of years, people had looked at the sky with their eyes – first on their own, and then through telescopes. But when Lovell and others started to look at the Universe with radio eyes, they saw completely new things. Instead of the stars in the night sky, a radio telescope sees the stuff between the stars, the radio waves produced by electrons spiralling around the magnetic field of the Galaxy.
Scattered across the sky we see what look like stars, but these turn out to be bright points of radio light from the regions around supermassive black holes at the centres of other galaxies.
The galaxy Cygnus A – at a distance of 750 million lightyears – viewed at three different wavelengths, all at the same scale, showing foreground stars and the central galaxy (top); radio jets and lobes extending about half a million lightyears (middle); and the hot gas enveloping the galaxy (bottom)
One of these ‘radio stars’, the galaxy Cygnus A (pictured above) is in the constellation of Cygnus, The Swan, and it’s a good example of the power of multiwavelength observations. Despite it being one of the brightest objects in the radio sky, there was nothing obvious when the astronomers looked at that position with optical telescopes. Just a faint
fuzzy nebulosity was visible; at the time it was thought that this might be two galaxies colliding.
Unfortunately, the view with a single radio telescope is blurred and a source like Cygnus A just looks like a large featureless blob. In order to get a sharper view, radio astronomers began connecting together multiple telescopes separated by large distances – a technique known as interferometry. This method overcame this limitation, resulting in the sharpest views available to astronomy. (A recent example is the Event Horizon Telescope’s image of the supermassive black hole in M87.) By using this approach in the 1950s, Roger Jennison and Mrinal
Dasgupta showed that the visible nebulosity of Cygnus A was situated between two bright sources of radio emission.
Modern views of Cygnus A, and other radio galaxies, show that the two radio lobes are produced by jets of material shooting from the centre of a galaxy in opposite directions. We can now look back and see that these remarkable objects remained undiscovered until we looked at the sky with radio eyes.
Whenever we look at the sky at a different wavelength, we discover something new, another piece in the cosmic jigsaw puzzle whose full picture is still emerging.
one of the meteors didn’t have a straight trail. I’d heard of these meteors before, but never seen one. I thought this might be a great example for showing how differences in meteor shape and aerodynamics
– and in layers of air – can affect their path as they pass through the atmosphere. Omid Qadrdan, Iran for storms all night was disappointing. Sure enough, as darkness set in, rain started to fall. We retired to the yurt, but I later ventured out to see stars twinkling in half of the sky and an immediate shooting star, followed by a flash of lightning from a storm to the north. I lay on a bench, fascinated by the meteor shower display in one half of the sky and the lightning storm in the other. Sure, this prevented my eyes from fully adjusting, but it didn’t stop me from seeing more shooting stars than I’d ever seen before, and the distinct smudge of the Milky Way. When I got home, I was still in awe of nature’s display and felt inspired to clean and collimate my telescope, brush up on my astronomy and dust off my old issues of BBC Sky at Night Magazine to plan a checklist for the rest of 2020!
Andrew Holmes, via email