Podcaster: Richard Drumm

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Title: UNAWE Space Scoop – This cosmic eruption could help planets grow

Organization: 365 Days Of Astronomy

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Description: Space scoop, news for children. 

A team of astronomers has just found a bright, uh, “spider” of a sort, in the sky.

It’s a very unusual binary system, a system in which two stars orbit each other, and it’s some 2,600 light-years away from us in the southern end of the constellation Crater, the cup.

Bio: Richard Drumm is President of the Charlottesville Astronomical Society and President of 3D – Drumm Digital Design, a video production company with clients such as Kodak, Xerox and GlaxoSmithKline Pharmaceuticals. He was an observer with the UVa Parallax Program at McCormick Observatory in 1981 & 1982. He has found that his greatest passion in life is public outreach astronomy and he pursues it at every opportunity.

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This is the 365 Days of Astronomy Podcast. Today we bring you a new episode in our Space Scoop series. This show is produced in collaboration with Universe Awareness, a program that strives to inspire every child with our wonderful cosmos.

Today’s story is…

This ‘cosmic spider’ spews gamma-rays!

A team of astronomers has just found a bright, uh, “spider” of a sort, in the sky.

It’s a very unusual binary system, a system in which two stars orbit each other, and it’s some 2,600 light-years away from us in the southern end of the constellation Crater, the cup.

This binary system is unlike almost any other because one of the stars, about to become a white dwarf, is orbiting a neutron star that may be a pulsar! 

Astronomers nicknamed binary systems like these “spiders” because the pulsar usually eats the outer parts of its neighbor as it turns into a white dwarf. Now, I say “pulsar” because that’s what they’ve found so far, pulsars, not just inert, non-pulsing, ordinary neutron stars.

Huh. As if neutron stars could be ordinary.

The system is called 4FGL J1120.0-2204. Let’s call it J1120 for short.

Of the gamma-ray sources that have not yet been categorized, it’s the second brightest.The astronomers didn’t know it was a binary system until just recently.

The Fermi Gamma-ray Space Telescope that NASA launched in 2008 had discovered J1120, but it hadn’t been categorized yet, so it was a bit mysterious. It was just a hot spot in gamma rays in the sky.

Using the 4.1-meter SOAR Telescope at Cerro Pachón in Chile, and its Goodman Spectrograph, a team of astronomers got a closer look in visible light at this mysterious source of gamma rays. 

I mean, you’ve gotta go look at it & see what it is!

The spectra they got showed them that the visible light was coming from a star with a surface temperature of 15,000°F and a mass just 17% that of our Sun. This points to it being a white dwarf.

But the strong gamma-rays and X-rays typically come from a neutron star or a pulsar of some sort, not a white dwarf. The astronomers analyzing its visible light spectrum found that the light would shift from red to blue over a 15.1 hour period. 

Objects that are moving toward us appear bluer, while objects that are moving away from us appear redder due to the Doppler effect. So this red to blue to red cycling shows that the visible light is coming from something that’s in a tight orbit.

But neutron stars are extremely massive and unlikely to be jerked around by something like this visible light source was.

It just got curiouser and curiouser!

The only possibility left is that the visible light they detected didn’t come from the gamma-ray source, but from a lower mass companion star that was in orbit around the neutron star that was the real gamma-ray source. 

They suspect that it’s an MSP, a millisecond pulsar, which is a pulsar that spins extremely fast, hundreds of times a second, as that’s what the other systems like this have in the center. 

MSP spin rates are measured in milliseconds, hundreds of revolutions every second! 

They spin as fast as the blades in a blender!

The source of the visible light was a late-stage red giant star that’s in the process of becoming a very low-mass white dwarf. 

A white dwarf is the remnant of a star with a mass equal to or lower than our Sun. When a star like our Sun runs out of hydrogen, it starts fusing helium which continues the nuclear fusion that powers the star. 

Helium fusion runs hotter than hydrogen fusion, it puts out more energy. The core contracts and heats up while the increased radiation pressure from the now hotter core causes the star’s outer layers to swell up into a red giant. 

The outer regions of the star are slowly pushed away from the core by the intense radiation pressure coming from there.

The outer parts are literally pushed out of the gravity well of the star and all that is eventually left is the core, incredibly hot and naked. 

In many systems the red giant’s outer atmosphere drifts away to become a planetary nebula, and is caused to fluoresce or glow by the UV radiation coming from the remnant core.

But in this system’s case the gas from the outer parts of the star are drawn away into the neutron star, making it spin faster & faster due to the conservation of angular momentum.

The star will eventually become a white dwarf about the size of Earth at insane temperatures. The surface will be over 100,000° Celsius or 180,000° Fahrenheit! 

It will take something like two billion years for the white dwarf in our J1120 spider to evolve to that point.

Millisecond pulsars spew gamma rays and X-rays especially when the pulsar’s solar wind of relativistic speed plasma hits material emitted by its neighboring star. I say “relativistic” because the surface of an MSP can easily be rotating at a third the speed of light!

MSPs get their fuel by swallowing gas from a neighboring star. In this case its neighbor is about to become a white dwarf and is possibly losing its outer layers so fast that it might not even look like a red giant at all.

This discovery could be the missing link to our understanding of how binary systems evolve!

Hey, here’s a cool fact!

We know of about 80 extremely low-mass white dwarfs, but this is the first one we’ve found orbiting a pulsar or a neutron star. 

The J1120 pulsar’s pulses haven’t been detected yet, so the astronomers will be using the GBT, the Green Bank Telescope, a radio telescope in West Virginia, to determine if the gamma-ray source is truly the pulsar that they suspect it is.

How did we discover pulsars?

In 1967, astronomer Jocelyn Bell Burnell was the first astronomer to detect a pulsar.

How did we discover white dwarfs?

In 1910, astronomer Willamina Fleming was part of the team that discovered white dwarfs. 

So is it dwarfs or dwarves?

Danged if I know! Ask Tolkien!

Thank you for listening to the 365 Days of Astronomy Podcast!

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365 Days of Astronomy

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