Orion is one of the more famous constellations, with its three belt stars, bright red-giant star Betelgeuse. When we observe Orion with the naked eye, we can see the bright Orion nebula (also known as M42) as a fuzzy patch within the sword of Orion. But the nebula we see is only the brightest region of a nebula that spans nearly the entire constellation, known as the Orion Molecular Cloud Complex.

The Orion Complex seen against a diagram of the constellation.

The Orion complex is about 240 light years across and only about 1,500 light years away, so it spans a fairly large region of sky. It is a large molecular cloud containing regions of reflection nebulae and emission nebulae, as well as dark nebulae such as the Horsehead nebula.

It is also a stellar nursery. Many of the stars seen in the constellation of Orion have their origins in the Orion complex. Most prominently, the three bright belt stars (Alnitak, Alnilam, and Mintaka) were formed within the cloud. The complex is one of the most active star production regions in the sky, and because of its proximity it gives us an excellent view of the process. When we view the region in infrared, we’ve found over 2,000 protoplanetary disks, where planets are likely forming around young stars.

There’s a lot going on in the region. But when we look at it with the naked eye, we simply see a bright, easy to find constellation. You could say the region is more complex than it seems.

The post Orion Complex appeared first on One Universe at a Time.

The Pillars of Creation is perhaps the most famous nebula in the night sky. One of the controversies regarding the pillars is whether they still exist. The nebula is about 7000 light years away from us, so what we see is the pillars as they were 7000 years ago. Observations of the Spitzer infrared telescope found evidence of a supernova shock wave, that by some calculations will destroy the pillars in about 6000 years. That would mean the pillars were destroyed about a thousand years ago. But we now know that isn’t the case, thanks to a bit of 3D imaging.

An infrared view of the pillars showing hot young stars in the nebula. Credit: NASA/ESA

The pillars were observed with an integral field spectrograph known as MUSE. This device allows us to measure the spectrum of an image at multiple points at the same time. From this we can determine not only where a particular pillar is in our field of view, but also how far away it is relative to other pillars. From this we can determine which parts of the pillars are behind the young O and B stars of the nebula, and which are in front. Using this data, a team of astronomers calculated the rate at which these bright stars are causing the pillars to evaporate. They found the pillars are deteriorating at a rate of about 70 solar masses per million years. Since the mass of the pillars is about 200 solar masses, it will be about 3 million years before they are completely destroyed.

That’s a short time on cosmic scales, but it also means that the Pillars of Creation are indeed still there, and will be for quite some time.

Paper: A. F. McLeod, et al. The Pillars of Creation revisited with MUSE: gas kinematics and high-mass stellar feedback traced by optical spectroscopy. MNRAS 450 (1): 1057-1076 (2015).

The post Depth of Field appeared first on One Universe at a Time.

The Boomerang Nebula is the coldest natural location in the universe. It has a temperature of 1 K, or just one degree above absolute zero. This is particularly interesting because the cosmic microwave background is about 3 K. That makes the nebula colder than empty space.

But how is it possible for this nebula to be colder than the universe? It turns out this nebula is transitioning from a red giant to a planetary nebula. A planetary nebula doesn’t have anything to do with planets, but rather is a gaseous nebula caused by a dying red giant. The term was first coined by William Herschel in the 1700s because he thought they looked like planetary systems starting to form, and the name stuck despite our improved understanding.

A planetary nebula forms when a sun-like star begins to run out of hydrogen to fuse for energy. The star begins fusing helium, which creates energy at a greater rate causing it to swell into a red giant. After its core can no longer fuse helium the star enters what is known as the asymptotic giant branch. While it can’t fuse hydrogen or helium in a continuous fashion, it is able to fuse them in bursts. This is similar to a gas engine running out of fuel, where it throttles up, dies down and throttles up again. These bursts of energy create tremendous stellar winds, which causes outer layers of gas to flow out from the star a high speed. In the case of the Boomerang Nebula, the outward flow is at about 160 km/s.

As gas expands it gets colder. You may have experienced this if you’ve used an aerosol can and felt the can get cold as you spray. In the same way, the outward flow of gas in the Boomerang Nebula causes the temperature to drop, which is how it can be colder than deep space. Such a low temperature is a product of the nebula’s dynamic evolution.

Recently in the Astrophysical Journal new observations of the nebula were presented. We’ve known about the nebula’s cold temperature for a while, and we’ve had good images of the nebula in the visible spectrum, such as the Hubble image above, but these new observations from the ALMA radio telescope array give us a high resolution radio image.

One of the interesting things the ALMA team found was that the nebula isn’t quite the shape seen in visible images. Many planetary nebula have a dual-lobed shape as seen in the visual images, but the Boomerang Nebula appears to have a more spherical shape. The reason the nebula appears lobed is because there is a thick dust region around the equatorial region of the star, which blocks light from the central star. As a result, only the polar regions are strongly illuminated from the central star, hence the lobed appearance.

While the Boomerang Nebula is currently the coldest known region in space, it won’t stay that cold forever. Already the outer envelope is starting to warm a bit due to its exposure to the (relatively) warmer surrounding space.

The post Cold Embers appeared first on One Universe at a Time.

E. E. Barnard is an astronomer perhaps best known for measuring the proper motion of a faint red dwarf about six light years away, now known as Barnard’s star. But Barnard was also a pioneer of astrophotography, and he did a great deal of work studying dark nebulae.

Dark nebulae are clouds of dust that are so dark they absorb most of the light in the visible spectrum. It takes astrophotography to study them, because they are only seen as shadows against background light from emission or reflection nebulae. Capturing their image takes long exposures, and Barnard was able to capture many of them in the early 1900s.

By 1919, Barnard had cataloged 182 dark nebulae, and published this in the Astrophysical Journal. He went on to catalog a total of 370 dark nebulae, now known as Barnard objects. Perhaps the most famous is B33, also known as the Horsehead nebula, seen above.

We now know that dark nebula contain dust grains that are nanometers to millimeters in size. This is why they scatter or absorb visible light. Their dark interiors can be quite cold (10s of Kelvin), which can allow for the formation of complex molecules. We can also see what is behind them in the radio spectrum, since they are generally transparent to radio waves.

But it was during the dawn of astrophotography that we began to understand these dark shadows in the night sky.

The post Dark Shadows appeared first on One Universe at a Time.

The Pillars of Creation (seen above) is an image of a portion of the Eagle nebula (M16) taken by Hubble Space Telescope in 1995. It soon became one of the most iconic space images of all time. The Eagle nebula is a stellar nursery, with several regions of gas and dust where stars are actively forming, including the pillars.

It’s hard to fathom the true size of the pillars. What looks like an image of dark clouds is actually about 10 light years across. If you look for the reddish star in the right pillar, and move left until you reach a reddish star in the middle pillar, that is roughly the distance from the Sun to Alpha Centauri (about 4 light years). So these pillars would span a large region of our local neighborhood. Of course the Pillars of Creation is just one small section of the much larger Eagle nebula, which spans more than 100 light years.

The Eagle nebula is only about 7000 light years away, so we are able to get a good view of this region. We can see regions where stars are being formed, and regions where gas and dust are being pushed away by intense radiation. Because of this, the pillars are shifting, though on a very slow time scale. For example, images from the Spitzer infrared telescope hint at a high velocity shock wave (possibly from a supernova) heading toward the pillars. Given the speed shock wave, it will begin to disperse the pillars in another millennia or so.

Of course, since the pillars are 7000 light years away, that would mean the shock wave dispersed the pillars about 6000 years ago. So depending on your definition of “now” it’s likely that the pillars no longer exist.

When we look at images such as the Pillars of Creation, they seem to be constant and unchanging, but in fact nebulae change incredibly fast on the cosmic scale. Within a few million years, stars will have formed, the remaining dust and gas will have been dispersed, and the Eagle nebula will be no more.

We’ve simply captured a moment of stellar creation in a very big universe.

The post Pillars of Creation appeared first on One Universe at a Time.

There’s a lot of gas and dust in the universe. Some of it has coalesced into dark nebulae, such as bok globules that almost look like holes in the starry night. We can observe these by the background light they absorb. Some clouds of dust are close enough to a star that light reflects off them, creating reflection nebulae such as the one near T Tauri. But sometimes a cloud of gas and dust is near a hot star, but too diffuse to scatter light much. In this case it can produce a faint nebula known as an emission nebula.

A typical emission nebula spectrum. Credit: Les Tomley

Since hydrogen is by far the most common element in the universe, these nebulae are mostly made up of hydrogen. When ultraviolet light from a nearby star strikes the hydrogen, the gas becomes partially ionized. When the hydrogen recombines, it emits light. Because the light emitted is mainly at red wavelengths, these emission nebulae typically appear as faint red clouds.

In astronomy these nebulae are often known as H II regions due to their quantities of ionized hydrogen. Because of their spectrum they can be identified in other galaxies, and since they only appear near very bright (and thus short lived) stars, they can be used to measure stellar production rates in galaxies. When both the H II region and bright nearby stars can be observed, we can also get an idea of a galaxy’s distance.

A few of the brightest nearby H II regions can be seen with the naked eye, but they weren’t noticed until after the introduction of telescopes. With modern astrophotography we can get wonderful images of these faint hydrogen clouds. Most of the popular images of nebulae have brilliant reflection regions and sharply contrasting absorption regions. But the faint emission nebulae have a beauty all their own, and they help us better understand both our own galaxy and others.

The post Red Light District appeared first on One Universe at a Time.

The Orion Nebula has been in the news recently due to a new set of pictures such as this one from Astronomy Picture of the Day.  It is an image of high velocity “bullets.”

It can be a bit difficult to wrap your head around what is really going on here.   The small blue dots at the tip of these orange streams are the “bullets,” and each one is about 10x larger than our solar system.  These bullets are not solid objects but rather dense lumps of iron-rich gas.  They’ve probably been pushed away from the central region of the nebula by intense stellar winds, and they now create shock waves as they stream through the surrounding nebular gas.  They are moving at about 400 km/s, ripping through the gas as they go.

We tend to think of nebula as static clouds of gas floating peacefully in space, but many nebulae are actually violent and complex interactions of gas, dust and stars.  Within the Orion Nebula, there is a cluster of bright central stars, there are dust masses colliding, and new stars are being born with planetary systems forming around them. There’s intense stellar wind colliding with all the gas and dust, pushing lumps like these bullets outward at supersonic speeds.

The Orion Nebula is not a particularly unusual nebula, but it is only about 1300 light years away, so it gives us a front row seat to the type of nebula that gives birth to stars.  Our own Sun likely formed in a similar stellar nursery billions of years ago.  So in a way this is like looking at baby pictures of your cousins.

The post Violence Inherent in the System appeared first on One Universe at a Time.