Category: Astronomy

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New maps of the moon from Japan’s Kaguya (SELENE) satellite suggest a lunar surface too rigid to allow for any liquid water, even deep below.

The new view is unveiled in one of three new papers in this week’s issue of the journal Science based on Kaguya (SELENE) data. In it, lead author Hiroshi Araki, from the National Astronomical Observatory of Japan, and international colleagues report that the Moon’s crust seems to be relatively rigid compared to Earth’s and may therefore lack water and other readily evaporating compounds. The new map is the most detailed ever created of the Moon, and reveals never-before-seen craters at the lunar poles.

“The surface can tell us a lot about what’s happening inside the Moon, but until now mapping has been very limited,” said C.K. Shum, professor of earth sciences at Ohio State University, and a study co-author. “For instance, with this new high-resolution map, we can confirm that there is very little water on the Moon today, even deep in the interior. And we can use that information to think about water on other planets, including Mars.”

Using the laser altimeter (LALT) instrument on board the Japanese Selenological and Engineering Explorer (SELENE) satellite, Araki and his colleagues mapped the Moon at an unprecedented 15-kilometer (9-mile) resolution. The map is the first to cover the Moon from pole to pole, with detailed measures of surface topography, on the dark side of the moon as well as the near side. The highest point — on the rim of the Dririchlet-Jackson basin near the equator — rises 11 kilometers (more than 6.5 miles) high, while the lowest point — the bottom of Antoniadi crater near the south pole — rests 9 kilometers (more than 5.5 miles) deep. In part, the new map will serve as a guide for future lunar rovers, which will scour the surface for geological resources.

But the team did something more with the map: they measured the roughness of the lunar surface, and used that information to calculate the stiffness of the crust. If water flowed beneath the lunar surface, the crust would be somewhat flexible, but it isn’t, the authors say. They add that the surface is too rigid to allow for any liquid water, even deep within the Moon. Earth’s surface is more flexible, by contrast, with the surface rising or falling as water flows above or below ground. Even Earth’s plate tectonics is due in part to water lubricating the crust.

Araki and his team say Mars, on a scale of surface roughness, falls somewhere between the Earth and the Moon — which suggests there may have once been liquid water, but that the surface is now very dry.

In the second Kaguya/SELENE study, lead author Takayuki Ono of Japan’s Tohoku University and colleagues describe debris layers between the near-side basalt flows, which suggest a possible period of reduced volcanism in the Moon’s early history. They propose that global cooling was probably a dominant driver of the shaping of lunar maria on the moon’s near side starting about 3 billion years ago. 

The third paper was authored by Noriyuki Namiki of Japan’s Kyushu University and his colleagues, who report gravity anomalies across the Moon’s far side indicating a rigid crust on the far side of the early Moon, and a more pliable one on the near side.

Source: Science

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Ancient astronomers thought that the Sun was a ball of fire, but now astronomers know that it’s nuclear fusion going on in the core of stars that allows them to output so much energy. Let’s take a look at the conditions necessary to create nuclear fusion in stars and some of the different kids of fusion that can go on.

The core of a star is an intense environment. The pressures are enormous, and the temperatures can be greater than 15 million Kelvin. But this is the kind of conditions you need for nuclear fusion to take place. Once these conditions are reached in the core of a star, nuclear fusion converts hydrogen atoms into helium atoms through a multi-stage process.

To complete this process, two hydrogen atoms are merged together into a deuterium atom. This deuterium atom can then be merged with another hydrogen to form a light isotope of helium – ³He. Finally, two of the helium-3 nuclei can be merged together to form a helium-4 atom. This whole reaction is exothermic, and so it releases a tremendous amount of energy in the form of gamma rays. These gamma rays must make the long slow journey through the star, being absorbed and then re-emitted from atom to atom. This brings down the energy of the gamma rays to the visible spectrum that we see streaming off the surface of stars.

This fusion cycle is known as the proton-proton chain, and it’s the reaction that happens in stars with the mass of our Sun. If stars have more than 1.5 solar masses, they use a different process called the CNO (carbon-nitrogen-oxygen) cycle. In this process, four protons fuse using carbon, nitrogen and oxygen as catalysts.

Stars can emit energy as long as they have hydrogen fuel in their core. Once this hydrogen runs out, the fusion reactions shut down and the star begins to shrink and cool. Some stars will just turn into white dwarfs, while more massive stars will be able to continue the fusion process using helium and even heavier elements.

We have written many articles about stars here on Universe Today. Here’s an article about a star that recently shut down its fusion reactions, and here’s a star that re-ignited its fusion reactions.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
http://www.jet.efda.org/fusion-basics/what-is-fusion/
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/procyc.html
http://large.stanford.edu/courses/2011/ph241/olson1/

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In today’s 365 Days of Astronomy podcast, two astronomers from the University of Minnesota discuss Eta Carina, a relatively close enigmatic star in the Carina Nebula. In a sense of great timing, new images also released today from the ESO (European Organisation for Astronomical Research in the Southern Hemisphere) reveal amazing detail in the intricate structures of the Carina Nebula, one of the largest and brightest nebulae in the sky. In addition to the gorgeous picture above, enjoy a pan-able image and a video that zooms in on this nebula (also known as NGC 3372), where strong winds and powerful radiation from an armada of massive stars are creating havoc in the large cloud of dust and gas from which the stars were born.

The Carina Nebula is located about 7,500 light-years away in the constellation of the same name (Carina; the Keel). Spanning about 100 light-years, it is four times larger than the famous Orion Nebula and far brighter. It is an intensive star-forming region with dark lanes of cool dust splitting up the glowing nebula gas that surrounds its many clusters of stars.

The glow of the Carina Nebula comes mainly from hot hydrogen basking in the strong radiation of monster baby stars. The interaction between the hydrogen and the ultraviolet light results in its characteristic red and purple color. The immense nebula contains over a dozen stars with at least 50 to 100 times the mass of our Sun. Such stars have a very short lifespan, a few million years at most, the blink of an eye compared with the Sun’s expected lifetime of ten billion years.

One of the Universe’s most impressive stars, Eta Carinae, is found in the nebula. It is one of the most massive stars in our Milky Way, over 100 times the mass of the Sun and about four million times brighter, making it the most luminous star known. Eta Carinae is highly unstable, and prone to violent outbursts, “In the 1840’s it blew up, and for about ten years it was one of the brightest stars in the sky,” said Dr. Kris Davidson in today’s 365 Days of Astronomy Podcast, hosted by Michael Koppelman of Slacker Astronomy. “But it’s almost a thousand times farther away than the brightest star in the sky Sirius, which means the amount of light coming out was really prodigious. After awhile it faded, now we see a nebula blowing out, expanding around it. Clearly its the ejecta the from the star. We can now ‘weigh’ the ejecta, and it is about 10 times the mass of the sun. That’s just the ejecta, the material the star lost about 160 years ago…. We have no right to have such a rare object that close!”

The large and beautiful image displays the full variety of this impressive skyscape, spattered with clusters of young stars, large nebulae of dust and gas, dust pillars, globules, and adorned by one of the Universe’s most impressive binary stars. It was produced by combining exposures through six different filters from the Wide Field Imager (WFI), attached to the 2.2 m ESO/MPG telescope at ESO’s La Silla Observatory, in Chile.

Source: ESO, 365 Days of Astronomy

Look up into the night sky and you’ll see stars dozens and even hundreds of light-years away. It’s hard to know where are the closest and which are the most distant stars because the brightest stars can be seen far away. Astronomers have measured the distance to most of the stars you can see with your unaided eye to determine which are the nearest stars.

Here is a list of the 20 closest star systems and their distance in light-years. Some of these have multiple stars, but they’re part of the same system.

1. Alpha Centauri – 4.2
2. Barnard’s Star – 5.9
3. Wolf 359 – 7.8
4. Lalande 21185 – 8.3
5. Sirius – 8.6
6. Luyten 726-8 – 8.7
7. Ross 154 – 9.7
8. Ross 248 – 10.3
9. Epsilon Eridani – 10.5
10. Lacaille 9352 – 10.7
11. Ross 128 – 10.9
12. EZ Aquarii – 11.3
13. Procyon – 11.4
14. 61 Cygni – 11.4
15. Struve 2398 – 11.5
16. Groombridge 34 – 11.6
17. Epison Indi – 11.8
18. Dx Carncri – 11.8
19. Tau Ceti – 11.9
20. GJ 106 – 11.9

So how do astronomers measure the distance to stars? They use a technique called parallax. Do a little experiment here. Hold one of your arms out at length and put your thumb up so that it’s beside some distant reference object. Now take turns opening and closing each eye. Notice how your thumb seems to jump back and forth as you switch eyes? That’s the parallax method.

To measure the distance to stars, you measure the angle to a star when the Earth is one side of its orbit; say in the summer. Then you wait 6 month, until the Earth has moved to the opposite side of its orbit, and then measure the angle to the star compared to some distant reference object. If the star is close, the angle will be measurable, and the distance can be calculated.

You can only really measure the distance to the nearest stars this way. This technique only works to about 100 light-years.

We have written many articles about stars here on Universe Today. Here’s an article about how new stars were discovered using the parallax method, and a newly discovered star that could be the third closest.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

Most stars have scientific names, but some have common names that have been passed down through history. Some astronomers use the scientific name, while others use the common name. Here’s a list of the brightest stars in the sky:

1. Sirius
2. Canopus
3. Arcturus
4. Alpha Centauri A
5. Vega
6. Rigel
7. Procyon
8. Achernar
9. Betelgeuse
10. Hadar (Agena)
11. Capella A
12. Altair
13. Aldebaran
14. Capella B
15. Spica
16. Antares
17. Pollux
18. Fomalhaut
19. Deneb
20. Mimosa

Of course, this is just a tiny list of stars. There are some enormous lists of stars out there. One of the most comprehensive is the SIMBAD database. This is an online database that contains 4.3 million objects. NASA has an even larger database of extragalactic objects that contains 163 million objects.

Here’s a good list of all the named stars in alphabetical order.

We have written many articles about stars here on Universe Today. Here’s an article about the biggest star in the Universe, and here’s an article that describes how massive stars form.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

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Look into the night sky and you’ll see stars in all directions shining and twinkling in the dark. But what is the light that we’re seeing, and how does it get all the way from the distant stars to here?

All stars are just vast balls of hot plasma. They’re made up of mostly hydrogen and helium, with trace amounts of other elements. Mutual gravity holds the star together, and compresses it inward. Without some kind of force pushing back, stars would just compress themselves down to the size of the Earth, or even smaller.

But as a star gets smaller, the gravitational friction causes it to heat up in its core. When the core of the star reaches about 15 million Kelvin, hydrogen fusion can begin. In this process, atoms of hydrogen are crushed together through a multi-stage process to form helium. This reaction is exothermic, which means that it gives more energy than it gives off. A star like the Sun is releasing 3.86 x 10²⁶ joules of gamma radiation every second.

These photons of energy are trapped inside the star and have to get out. Over a journey that can take more than 100,000 years, the photons are continuously emitted and then absorbed by atoms in the Sun. Each of these jumps can cause the photon to lose energy. When they finally reach the surface of the star, they’ve lost a tremendous amount of energy, and have fallen from high energy gamma rays down to visible wavelengths.

And then, the photons are released from the surface of the star, and free to cross the vacuum of space. Unless they encounter anything, they’ll keep traveling in a straight line for millions, billions and even trillions of years. When you step outside and look at a star that could be a few hundred light-years away, your eyes are the first things the photons have bumped into since they left the surface of the star!

We have written many articles about stars here on Universe Today. Here’s an article about the biggest star in the Universe, and here’s an article about how many stars there are in the Milky Way.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
http://www.jet.efda.org/fusion-basics/what-is-fusion/
http://www.ips.gov.au/Category/Educational/The%20Sun%20and%20Solar%20Activity/General%20Info/Solar_Constant.pdf

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The vast majority of stars out there are tiny red dwarfs, then come the solar mass stars like our Sun. There are giant stars and even supergiant stars. But the biggest stars out there are the monstrous hypergiant stars, which pump out millions of times more energy than the Sun. So just how big and powerful are hypergiant stars?

First, let’s take a look at a regular star like our Sun. Our Sun is the baseline, with 1 solar mass, and 1 solar diameter. It puts out 1 solar amount of luminosity. An example giant star would be Rigel, with 17 times the mass of the Sun. It’s putting out about 66,000 times as much energy as the Sun, and it’s estimated to have 62 times the radius of the Sun.

Next, let’s go bigger and look at a supergiant star: Betelgeuse. This familiar star is located in the constellation Orion, and has 20 times the mass of the Sun (1 solar mass = the mass of the Sun). Betelgeuse is estimated to be 1000 times the size of the Sun, and puts out 135,000 times as much energy.

Those stars are nothing compared to hypergiant stars. An example of a red hypergiant star is VY Canis Majoris, which measures 1,500 times the size of the Sun.

The true monsters of the Universe are the blue hypergiant stars, like Eta Carinae. It has 150 times the mass of the Sun, and measure up to 180 times the size of the Sun. Eta Carinae is putting out 4 million times as much energy as the Sun! Of course, Eta Carinae is a “live fast, die young” kind of star. It’s probably only been around for 3 million years or so, and astronomers think it’ll detonate as a supernova within 100,000 years.

We have written many articles about stars here on Universe Today. Here’s an article about the biggest star in the Universe, and here’s an article about Eta Carinae, which is expected to blow up any time now.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
http://en.wikipedia.org/wiki/VY_Canis_Majoris
http://en.wikipedia.org/wiki/Rigel
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970616b.html
http://seds.org/messier/xtra/ngc/etacar.html