Category Archives: AAS January Meeting 2011

By Hamish Johnston at the AAS meeting in Seattle

The jetlag and non-stop astronomy must be getting to me because I can’t stop thinking about various aspects of astrophysics in terms of my favourite sitcoms.

For example, Father Ted brings us the “Dougal effect”, whereby the actual size of an astronomical object cannot be inferred from its observed size alone. Distance must also be considered and Ted explains this to Dougal using nearby toy cows and a distant herd of real cows. “These are small… but the ones out there are far away,” is the best way to define the effect.

Then there’s that astronomical unit of temperature defined in I’m Alan Partridge. Alan uses a microwaved apple pastry as a weapon, discovering “It’s hotter than the Sun”. To calibrate your thermometer to one “Partridge” put a petrol-station pastry in the microwave for eight minutes and presto.

Well, that’s all from me in Seattle. I’m about to fly back to Blighty and I’ll be looking for astronomy references in The Inbetweeners, which is featured on the in-flight entertainment system.

A supermassive black hole could look like this: but how did it form? (Courtesy: NASA)

By Hamish Johnston at the AAS meeting in Seattle

The universe is full of supermassive black holes (SBHs). Indeed, they make up the core of just about every galaxy. These monstrosities can be a billion times more massive than the Sun. But despite their size and ubiquity, astrophysicists don’t really understand how they are formed.

That was the topic of a fascinating talk by Mitch Begelman of the University of Colorado, who is an expert on SBH formation.

According to Begelman there are two competing theories – the small seed that takes a long time to grow, and the large seed that grows quickly.

The small seed refers to the collapse of a massive star of about 100–1000 solar masses to form a black hole that grows slowly by sucking in surrounding gas and merging with other structures until it is an SBH.

The large seed refers to the direct collapse of a huge cloud of gas to create a supermassive star that could be heavy as a billion Suns. According to Begelman, such stars would be very fragile and would only last a few million years until their cores collapsed to create a black hole.

But instead of exploding in a supernova like much smaller stars, the remaining matter would puff out to become a “quasistar” – resembling a red giant. This surrounding matter is rapidly sucked in and what remains is a black hole that Begelman believes could be as large as one million solar masses. This is around the lower limit of an SBH, and it could keep growing.

Sounds great, but is there any chance of seeing a supermassive star or quasistar?

Unlikely for supermassive stars, says Begelman, because they would be very hard to distinguish from clusters of hot stars. He is a bit more hopeful about quasistars, because they could stand out in the optical and infrared wavelengths. However, he concedes that this would be a tough job, even with the upcoming launch of the James Webb Space Telescope.

To paraphrase Begelman’s conclusion, SBH formation models are getting more sophisticated but the problem has not yet been solved.

By Hamish Johnston at the AAS meeting in Seattle

You know it has been a good day when you learn a new amazing fact.

Today I discovered that the Moon is brighter than the Sun when it comes to gamma radiation.

How can a cold lump of rock give off more gamma radiation than a seething fusion reactor?

The answer, according to NASA’s Julie McEnery, is that both bodies glow with gammas because they are illuminated by cosmic radiation. The Sun’s strong magnetic field deflects much of this radiation away from the star. The Moon, however, has an extremely weak field that is not much use at deflecting cosmic rays.

By Hamish Johnston at the AAS meeting in Seattle

Everyone likes a good thunderstorm – the spectacular flashes, crashes and wind, and then calm, clear air. But for the past few years physicists have begun to realize that thunderstorms can generate very-high-energy gamma rays – 100 MeV being the highest seen so far.

Now, researchers have discovered that these gamma rays are creating beams of positrons (the antimatter version of electrons) and hurling them into space!

These bizarre discoveries have come about thanks to the Fermi gamma ray telescope – the primary mission of which is to scan the heavens for gamma ray bursts. However, the satellite can’t help detecting terrestrial gamma ray bursts (TGBs) and once it determined that they are a regular occurrence over the tropics it was optimized to look down as well as up.

While most TGBs last about a millisecond, Fermi has seen events that last for much longer. What’s more intriguing is that one of these events appeared to occur over southern Egypt, where there was no thunderstorm activity.

Loving a good mystery, Michael Briggs at the University of Alabama and colleagues decided to investigate. One thing that they noticed was a preponderance of gamma rays at 511 keV, which are produced when electrons and positrons annihilate.

The positrons are created when high-energy gamma rays scatter off atoms in the atmosphere, converting into electron–positron pairs. Even more electrons are created by other scattering processes and, being charged particles, the electrons and positrons travel along Earth’s magnetic field lines. As they travel, they collide with gas atoms and can emit gamma rays.

So what does this have to do with the TGB over Egypt? What Briggs and colleagues think is that the initial TGB occurred thousands of miles away in southern Africa, sending a beam of electrons and positrons hurling up and over Egypt, where collisions produced gamma rays. See the above figure.

The charged particles kept going to a mirror point, where they were reflected back down over Egypt – creating a second pulse. This entire process took less than 30 ms.

Amazingly, physicists have only known about these high-energy bursts for a decade or so. The problem, according to Briggs, is that they are difficult to detect here on Earth because the gamma rays are absorbed by the dense lower atmosphere. However, they have been seen at certain high-altitude facilities and at sea level in Japan, where thunderstorms are believed to occur lower in the sky than in most places.

The big mystery, however, remains how such high-energy gamma rays are created in the first place.

By Hamish Johnston at the AAS meeting in Seattle

Times are tough, and cutting costs was on the agenda for the two speakers who opened the 217th Meeting of the American Astronomical Society here in Seattle.

On the podium first was AAS president Debra Elmegreen, who had something to say about the cost of coffee at the Seattle Convention Center – which is astronomical. Indeed, catering is the single largest expense for the meeting, and free coffee adds over one hundred dollars per delegate. Yikes, that’s a lot of money considering that more than 10% of the 2700 folks here are undergraduates.

So no more free coffee breaks between sessions – with the exception of ticketed coffee in the exhibition – and not a groan in the audience. If only the bankers would take the same attitude towards their bonuses!

The next speaker was the Nobel laureate John Mather, who spoke about progress towards launching the James Webb Space Telescope in 2015. The big news is that construction of the telescope’s 18 primary mirrors is well under way and they should all be completed by this summer. “It’s huge,” said Mather, referring to the telescope, which is 6.5 m across, compared with Hubble at 2.4 m.

The next big step, according to Mather, is to place the entire optical system into a giant chamber at the Johnson Space Center to simulate the rigours of space.

Mather took a few questions, which is when the thorny issue of money came up. A recent article in the New York Times pointed out that cost overruns on the James Webb were going to sap funds from other NASA missions. Not surprisingly, Mather was adamant that the extra funds were needed, and expensive projects such as the Johnson testing must go ahead.

I’m afraid that at this point there was some muttering in the audience – and the person next to me said under his breath that the James Webb was consuming far too much of NASA’s astrophysics budget.

Let’s hope Mather and colleagues can keep costs under control for the sake of my neighbour’s blood pressure.