Posted on by Ian O'Neill

Where Have All the Gamma Ray Bursts Gone?

Artist impression of a GRB (ESA)

[/caption]

Astronomers are confused.

As if gamma-ray bursts (GRBs) weren’t mysterious enough, there’s something else to add to the bag of confusion. GRB events are missing from the furthermost reaches of the Universe. Right around the time when there should be a lot of GRBs, during the “star forming epoch” (when stars were just beginning to evolve after the Big Bang), there appears to be none. Zero. There’s no ancient flashes of massive star death to be found. What’s more, there doesn’t appear to be any afterglow from previous gamma-ray bursts either.

So what’s going on? Were there no GRB events before 12.8 billion years ago? Possibly… although there might be another answer. They are out there but we just can’t see them.

Gamma-ray bursts are the biggest and brightest explosions in our Universe since the Big Bang. When a GRB detonates, it can easily outshine its host galaxy containing billions of stars. These energetic events have been observed since the 1960’s and only until recently have astronomers found an explanation as to what GRBs are. A GRB occurs when a young metal-poor massive star has used up all its fuel and, like a supernova, collapses under its own gravitational field. The rapid-spinning star then funnels intense beams of radiation from its poles in the form of gamma-rays. Should one of these beams be directed toward Earth, we see a disproportionately bright explosion (as a vast amount of energy is channelled through the poles). Until the “collapsar model” was devised, astronomers were at a loss to explain these energetic events.

The collapsar model appears to explain GRBs lasting for two seconds or more. However, there is another class of GRB, of much shorter timescales, that does not fit in with the collapsar model. Short-period GRBs may be the result of violent interactions between black holes and a neutron stars.

So, does this mean GRBs are becoming less mysterious? Actually, GRB theory has just become a little more complicated. It would appear that no GRBs occurred before 12.8 billion years ago. Last month, the most distant (and therefore oldest) GRB was detected 12.8 billion light years away, but that in itself is strange.

During the time when the first stars started to form (around 13.4 billion years ago), they were by definition “metal-poor” stars (heavier elements, such as metals, were only possible after several generations of stellar evolution), so this should be a period of time when GRBs were regularly lighting up the night sky. However, according to observations of the most distant galaxies containing the youngest stars, GRB events seem to be non-existent.

One explanation put forward is the effect of red shift. As the Universe expands, space-time stretches. As light travels from the most distant reaches of the Universe, perhaps the light itself from GRBs has been so stretched (red-shifted) that the electromagnetic emissions simply cannot be detected by our instrumentation. These huge explosions could be happening, but as the emitted light has been so red-shifted, by the time the light reaches us, perhaps the emission does not resemble a GRB. Even the afterglow of one of these massive explosions would be unrecognisable in this case, the light observed would be shifted all the way into the infrared.

So will any GRBs be discovered further away than 12.8 billion light years? I think we’ll have to wait until we build some improved infrared observatories or recognise what a distant, ancient GRB looks like…

Source: NASA

Posted on by Ian O'Neill

Asteroseismology: Observing Stars Vibrate with CoRoT

Modes of solar oscillation plotted over our Sun. Could the same things be done with other stars? (NASA/TRACE/NCAR)

[/caption]Observing a stars brightness pulsate may reveal its internal structure say researchers using the Convection Rotation and Planetary Transits (CoRoT) observatory. The highly sensitive orbital telescope can detect tiny variations in a distant star’s brightness, leading astronomers into a new field of stellar seismology called “asteroseismology.”

Seismology is more commonly used by scientists on Earth to see how waves travel through the terrestrial crust, thereby revealing the structure of the material below us. Even solar physicists use the method of helioseismology to understand the interior of our Sun by observing its wobble. Now, by observing the slight changes in stellar brightness, it is possible to remotely probe deep into the inner workings of a distant star…

CoRoT is a joint French Space Agency (CNES) and European Space Agency (ESA) mission to detect slight variations in the brightness of stars launched in 2006. As extrasolar planets pass in front of (or “transit”) a star, the brightness will decrease. The highly sensitive 27 cm-diameter telescope and spectroscopic instrumentation has the ability of detecting extrasolar rocky planets a few times the size of Earth and new gas giants (a.k.a. Hot Jupiters).

Another mission objective for the 630 kg satellite is to detect luminosity variations associated with acoustic pulsations passing through the body of the star. A similar method known as helioseismology uses the Solar and Heliospheric Observatory (SOHO) to detect the propagation of pressure waves through the Sun so a better idea of solar internal dynamics and structure can be gained.

CoRoT has been watching three stars, 20-40% more massive than the Sun, vibrate in reaction to the convective processes on the stellar surfaces. Some areas will expand and cool, whilst others with contract and heat up. This creates an oscillation, and a pulsation in brightness, providing information about the inner structure of these distant stars. The three stars brightened and dimmed 1.5 times more dramatically than solar helioseismology observations. However, this is still 25% weaker than expected from theory, so it would seem stellar physics still has a long way to go.

“This really marks the start of a completely new era of space-based asteroseismology,” said Joergen Christensen-Dalsgaard of the University of Aarhus in Denmark. “It shows that CoRoT can do what it set out to do.”

Asteroseismology can also be used to gauge the precise age of a star. Usually, the age of a star is determined by looking at a star cluster where it is assumed the majority of the stars are of a similar age. However, as a star ages, different elements undergo nuclear fusion at different times. This alters the star’s interior structure and therefore alters the vibrational characteristics of the star. This can be detected by CoRoT, hopefully aiding astronomers when deducing the precise ago of a particular star.

“In principle, you can look at one star all on its own and determine how old it is,” adds Michael Montgomery of the University of Texas.

Source: New Scientist

Posted on by Fraser Cain

Moon Albedo

Earthshine on the Moon. Image credit: Drew J. Evans

[/caption]
The albedo of the Moon is 0.12. In other words, the Moon reflects back 12% of all the radiation that falls upon it.

As you may or may not know, albedo is a term that astronomers use to measure reflectivity of an object in space; more specifically, it measures how much of the Sun’s radiation an object reflects. An albedo of 0 means that object is dark, while a 1 means that it’s very bright and reflective.

So, how does the Moon albedo compare to other objects in the Solar System? As bright as the Moon looks from our perspective here on Earth, the Moon’s albedo is actually pretty low. The object with the highest albedo in the Solar System is Saturn’s moon Enceladus, which has an albedo of 0.99, which means that it’s covered with very reflective snow and ice. The Moon is much more similar to a very dark object, like an asteroid. The darkest asteroids in the Solar System have an albedo of 0.06. That’s pretty close to 0.

The brightness of the Moon changes as its phases change. During the first and last quarters, the visible Moon is illuminated 50% by the Sun, but it only has about 8% of the brightness of a full Moon. This is because the sunlight is hitting the Moon at an angle and deflecting away from us. When the Moon is full, sunlight is hitting the Moon and then bouncing directly back. This is why the Moon is so much brighter during a full Moon.

We have done lots of articles about the Moon on Universe Today. Here’s an article about explosions on the Moon. And here’s another about building a Moon base.

Here’s a great article that helps explain the Moon’s albedo.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

Posted on by Fraser Cain

Light From the Moon

Gibbous Moon - Roger Warner

[/caption]
The light from the Moon is actually the reflected light from the Sun!

Every second, the Sun is converting 600 million tons of hydrogen into helium. This reaction lets out a tremendous amount of energy. Most of this energy heads off into space, but some of it falls on the planets and the moons in the Solar System. This is why we can see them at all. Without the Sun, only Jupiter and Saturn would be readily detectable in the infrared spectrum, as they give off more heat than they absorb from the Sun.

Different objects in the Solar System have a different amount of reflectivity. Astronomers call reflected light from the Sun albedo. The values for an object’s albedo can range between 0 (dark) and 1 (bright). The albedo for the Moon is 0.12. In other words, the Moon reflects 12% of the sunlight that falls onto it.

The highest albedo in the Solar System is Saturn’s moon Enceladus, with an albedo of .99. In other words, it reflects 99% of the light that falls upon it. This is because it’s composed of mostly ice. Dark objects like asteroids can have an albedo down to 5%.

When the Moon is full, astronomers measure its apparent magnitude at -12.6. This is bright enough to easily walk around in otherwise total darkness; almost bright enough to read.

When there’s a new moon, however, there’s no light from the Sun falling on the Moon, and yet we can still see the surface of the Moon. Where’s the light coming from? The Earth. Astronomers call this reflected light “Earthshine”, and it helps them calculate how much sunlight is falling on the Earth.

So now you know that the light of the Moon actually comes from the Sun, unless it’s coming from the Earth.

Here’s an article we’ve done on Universe Today about Earthshine, and here’s an article about gardening on the Moon.

Here’s an article from Extreme Science about the Moon, and an article from NASA about Earthshine.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

Posted on by Fraser Cain

The Moon and Venus

The Moon, Venus and the Pleiades cluster

[/caption]
Except for the Sun, the Moon and Venus are the two brightest objects in the sky. You just can’t miss them.

Astronomers measure relative brightness of objects in the sky with a term called magnitude. The Sun has the highest magnitude of any object in the sky, at -26.73 apparent magnitude. The brightness of the Moon, for comparison, has a magnitude of -12.6. These numbers are done on a logarithmic scale. So the brightness of the Sun is actually 449,000 times brighter than the full moon.

The maximum brightness of Venus is -4.7 magnitude, which is a fraction of the brightness of the Moon (and the Sun!).

So why is the Moon so much brighter than Venus? It’s closer. The distance to the Moon is about 384,000 km, while the closest distance to Venus is about 38 million km. In other words, the Moon is about 100 times closer to Earth than Venus.

Both the Moon and Venus can cast shadows when they’re in the sky. The Moon can be bright enough to almost read by. It’s easy to navigate when the full moon is in the sky. When the Moon isn’t in the sky, but Venus is very bright, you have to look carefully to detect shadows cast by Venus.

One of the most amazing things in astronomy is when Venus and the Moon are both in the sky; especially when they’re very close to one another. Make sure you head outside to see the Moon and Venus with your own eyes.

We have done several articles informing readers about times when both the Moon and Venus are visible in the sky. Here’s an example, and here’s another.

And there are some great articles out there on the Internet. Here’s one from SPACE.com, and another from Astronomy Picture of the Day.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

Posted on by Fraser Cain

Flag on the Moon

Flag on the Moon. Image credit: NASA

[/caption]
When the NASA astronauts first landed on the Moon, they left a few items on the surface to commemorate their visit. These items included a plaque, mission badges and an American flag. If you’ve ever seen images or video of the flag on the Moon, you might have a few questions.

Why does the flag stand straight out and not just slump down? Here on Earth, flags are pushed out by the wind. Obviously, there’s no wind on the Moon, so what’s holding the flag up? The answer is pretty easy. There’s a rod, sort of the like a curtain rod running across the top. So the flag on the Moon is being held out by the rod and isn’t blowing in the wind.

What makes the flag flap if there’s no wind? You might have also see a few videos of the flag on the Moon waving back and forth. This happened when the astronauts first planted the flag. There’s no wind to make the flag to flag, but there’s also no wind to stop it from moving back and forth. When the astronauts planted the flag on the Moon, they couldn’t help but give it a sideways push. Without the wind resistance the flag would experience on the Earth, the flag can flap back and forth a few times before finally settling down. That’s why it looks like it’s flapping, even though there’s no wind.

There’s another scene where the flag flaps, as the lunar ascent module is taking off. In this case, the exhaust from the rocket is blasting the flag and causing it to flap back and forth. In the case of Apollo 11, the exhaust blast was so strong that the flag actually fell over. Later missions kept the flag much further away from the ascent rocket.

Can we see the flag on the Moon from Earth with a big telescope, or even Hubble? Even though we have some powerful telescopes, they’re just not powerful enough to spot objects the size of a flag on the surface of the Moon. The flag is only a meter across. In fact, you would need a telescope 200 meters across to spot objects that size from here on Earth. Future space missions will return to the Moon, and they should be able to resolve objects as small as the flags on the Moon.

Does the flag mean that the US claims the Moon? Nope, the Moon can’t be owned by anyone. NASA had the astronauts plant the flag to commemorate the journey made by American astronauts, but to not actually claim the Moon for any single nation.

We’ve done a few articles about this topic. Here’s a review of the Mythbusters episode where they debunk the Moon flag myth.

NASA has answered some more questions about the flag on the Moon. Here’s a link to their article. And here’s another article debunking the conspiracy theory that NASA didn’t even go to the Moon.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

References:
NASA Science: The Great Moon Hoax
NASA Apollo 11 Videos

Posted on by Fraser Cain

First Man on the Moon

Astronaut Buzz Aldrin, the second man on the Moon.

[/caption]
The 1st man on the moon was the Apollo 11 Commander Neil Armstrong, who made history on July 20, 1969.

The Apollo 11 mission consisted of Command Module Pilot Michael Collins, Lunar Module Pilot Buzz Aldrin and Commander Neil Armstrong. The mission launched atop a Saturn V rocket on July 16, 1969. After a 4 day journey from the Earth to the Moon, the lunar module detached from the command module and landed on the surface of the Moon in the southern Sea of Tranquility.

The crew remained inside the module for 6 and a half hours, preparing to make their exit onto the lunar surface. And then Neil Armstrong descended the ladder from the lunar module and onto the lunar surface. The first words spoken by the first man on the Moon were, “that’s one small step for (a) man, one giant leap for mankind.”

Buzz Aldrin followed Armstrong, and the two remained on the surface of the Moon for 2.5 hours, taking photographs, collecting rocks, drilling samples, and placing scientific experiments. They they gathered up all their samples, stowed them in the lunar module, and left some souvenirs on the surface of the Moon, like an American flag, Apollo 1 mission patch, and commemorative plaque. They launched again and returned to Earth on July 24.

After the 1st man on the Moon, Neil Armstrong, there were a total of 12 astronauts to walk on the surface of the Moon.

Want to experience what it might have been like to be the first man on the Moon? Here’s a movie review of Fly Me to the Moon.

Of course, NASA has a tremendous amount of information about Apollo 11. Here’s the NASA history page about Apollo 11. And here’s a page that was put together for the 30th anniversary of the first man on the Moon.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

Posted on by Fraser Cain

Area of the Moon

NASA's image of the Moon

[/caption]
The total surface area of the Moon is 37.9 million square kilometers, or 14.6 million square miles.

Need some context? The surface area of the Earth is 510 million square kilometers. In other words, the total surface area of the Moon is only 7.4% the surface area of the Earth. If you could unwrap the Moon and lay it out flat on the Earth, it wouldn’t fill up Asia, which has an area of 44.4 million square kilometers.

Wondering about some other aspects of the Moon? Here’s an article about the mass of the Moon, and here’s one about the diameter of the Moon.

Want some more general information about the Moon? Check out NASA’s Lunar and Planetary Science page. And here’s the Solar System Exploration Guide.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

Posted on by Nancy Atkinson

Fermi Telescope Makes First Big Discovery: Gamma Ray Pulsar

The pulsar lies in the CTA 1 supernova remnant in Cepheus. Credit: NASA/S. Pineault, DRAO

[/caption]
NASA’s Fermi Gamma-ray Space Telescope discovered the first pulsar that beams only in gamma rays. A pulsar is a rapidly spinning neutron star, the crushed core left behind when a massive sun explodes. Astronomers have cataloged nearly 1,800 pulsars. Although most were found through their pulses at radio wavelengths, some of these objects also beam energy in other forms, including visible light and X-rays. However, this new object only pulses at gamma-ray energies. “This is the first example of a new class of pulsars that will give us fundamental insights into how these collapsed stars work,” said Stanford University’s Peter Michelson, principal investigator for Fermi’s Large Area Telescope.

The gamma-ray-only pulsar lies within a supernova remnant known as CTA 1, which is located about 4,600 light-years away in the constellation Cepheus. Its lighthouse-like beam sweeps Earth’s way every 316.86 milliseconds. The pulsar, which formed about 10,000 years ago, emits 1,000 times the energy of our sun.

“We think the region that emits the pulsed gamma rays is broader than that responsible for pulses of lower-energy radiation,” explained team member Alice Harding at NASA’s Goddard Space Flight Center in Greenbelt, Md. “The radio beam probably never swings toward Earth, so we never see it. But the wider gamma-ray beam does sweep our way.”

Scientists think CTA 1 is only the first of a large population of similar objects.

“The Large Area Telescope provides us with a unique probe of the galaxy’s pulsar population, revealing objects we would not otherwise even know exist,” says Fermi project scientist Steve Ritz, also at Goddard.

Watch an animation of pulsar.

Fermi’s Large Area Telescope scans the entire sky every three hours and detects photons with energies ranging from 20 million to more than 300 billion times the energy of visible light. The instrument sees about one gamma ray every minute from CTA 1, enough for scientists to piece together the neutron star’s pulsing behavior, its rotation period, and the rate at which it is slowing down.

The pulsar in CTA 1 is not located at the center of the remnant’s expanding gaseous shell. Supernova explosions can be asymmetrical, often imparting a “kick” that sends the neutron star careening through space. Based on the remnant’s age and the pulsar’s distance from its center, astronomers believe the neutron star is moving at about a million miles per hour — a typical speed.

Source: NASA

Posted on by Ian O'Neill

Feeding Time at the Stellar Zoo: Infant Stars Generate Lots of Gas

Artist's impression of a young star with surrounding disk of dust (ESO/L. Calçada)

[/caption]

Understanding how stars form is critical to astronomers. If we can gain a better understanding of how intermediate-size infant stars grow, we can begin to answer some of the most perplexing questions hanging over the evolution of our own Solar System. Unfortunately, the nearest star forming regions are about 500 light years away, meaning that astronomers cannot simply use traditional optical telescopes to peer into star-forming disks of gas and dust. So, researchers working with the European Southern Observatory (ESO) are combining high resolution spectroscopic and interferometry observations to give the most detailed view yet of infant stars eating away at their proto-planetary disk, blasting out violent stellar winds as they do so…

It sounds like baby stars are very much like their human counterparts. They need a conveyor belt of food supplying their development and they blast huge amounts of waste back out in the form of gas. These findings come from researchers using the ESO’s Very Large Telescope Interferometer (VLTI), giving us milli-arcsecond resolution when focusing on these star-forming regions. The detail this provides is equivalent to studying the period (‘full stop’ as I prefer to call it) at the end of this sentence at a distance of 50 km (31 miles).

This high resolution is achieved by combining the light from two or more telescopes separated by a certain distance. This distance is known as the “baseline,” and interferometers such as the VLTI have a large baseline (of up to 200 metres), simulating a telescope diameter equivalent to this distance. However, the VLTI now has another trick up its sleeve. The AMBER spectrometer can be used in conjunction with the interferometer observations to give a more complete view of these feeding stars, probing deep into the spectrum of light being emitted from the region.

“So far interferometry has mostly been used to probe the dust that closely surrounds young stars. But dust is only one percent of the total mass of the discs. Their main component is gas, and its distribution may define the final architecture of planetary systems that are still forming.” – Eric Tatulli, co-leader of the VLTI international collaboration from Grenoble, France.

The Herbig Ae/Be star R Coronae Australis, a young intermediate-size star (2MASS)
The Herbig Ae/Be star R Coronae Australis, a young intermediate-size star (2MASS)
Using the combined power of the VLTI and AMBER instrument, astronomers have been able to map this gas surrounding six stars belonging to the Herbig Ae/Be family. These particular stars are typically less than 10 million years old and a few times the mass of our Sun. They are very active stars in the process of forming, dragging huge amounts of material from a surrounding disk of dust.

Until now, astronomers have not been able to detect gas emission from young stars feeding on their stellar disks, thereby keeping the physical processes acting close to the star a mystery.

“Astronomers had very different ideas about the physical processes that have been traced by the gas. By combining spectroscopy and interferometry, the VLTI has given us the opportunity to distinguish between the physical mechanisms responsible for the observed gas emission,” says co-leader Stefan Kraus from Bonn in Germany. In two of the Herbig Ae/Be stars, there is evidence for a large quantity of dust falling into them, thereby increasing their masses. In four cases, there is evidence for a strong stellar wind, forming an extended stellar gas outflow.

The VLTI observations also reveal dust from the surrounding disk is much closer than one would expect. Usually there is a cut-off distance for dust location as the stars heat will cause it to vaporize. However, it would appear in one case that gas between the star and dusty disk shields the dust from evaporating; the gas acts as a radiation-block, allowing the dust to extend closer to the star.

“Future observations using VLTI spectro-interferometry will allow us to determine both the spatial distribution and motion of the gas, and might reveal whether the observed line emission is caused by a jet launched from the disc or by a stellar wind“, Kraus added.

These phenomenal observations of star-forming dust disks and gas emission, 500 light years away, open up a new kind of high-resolution astronomy. This will help us understand how our Sun fed off its surrounding disk of dust, eventually forming the planets and, ultimately, how life on Earth was possible…

Source: ESO