Rigel is the brightest star in the constellation of Orion; despite that, its formal name (one of them anyway) is Beta Orionis (Alpha Orionis – Betelgeuse – is a variable star, as is Rigel; Betelgeuse is sometimes the brighter, but most of the time is the fainter).

Rigel is a blue supergiant (spectral class B8I), the brightest of its kind in the sky. It’s also a multiple star system … the primary is the blue supergiant which totally dominates the observed light, and the secondary (Rigel B) is itself a close (spectroscopic) binary (B, and C, are both of B spectral class too … but are main sequence stars). HIPPARCOS data puts Rigel at a distance of ~850 light-years, but with a large uncertainty (GAIA will nail down its distance much more accurately).

Being a blue star, Rigel emits most of its light in the UV; if it is 850 light-years distant, its luminosity is approximately 85,000 sols, its radius ~75 sols (or ~0.35 au; if it were where the Sun is, Mercury would be almost inside it), its mass about 18 sols, and it is only approximately 10 million years old. It is likely to have a non-burning helium core (i.e. it is in its hydrogen shell-burning phase), and on its way to becoming a red supergiant (like Betelgeuse), and after that a supernova.

A couple of degrees away, on the sky, is the Witch-Head Nebula (IC 2118), which is a reflection nebula. And which star’s light is it reflecting? You guessed it, Rigel’s! Now as IC 2118 is about 40 light-years from Rigel, it demonstrates well just how much light Rigel is emitting.

Rigel may be part of the Orion OB1 association, if it were kicked out at around its birth (it’s too far, today, from the other stars in the association to be a member unless it is moving away at rather a fast clip).

Some of the Universe Today articles which feature Rigel include Rigel Passes Behind Saturn, Astrophoto: The Witch Head Nebula by Richard Payne, and IYA 2009 – Brian Sheen Reports on “Canoe Africa”.

Two Astronomy Cast episodes which relate to Rigel are The Life of Other Stars (in particular, the life of stars much more massive than the Sun), and Stellar Populations (in particular, the range of types of stars born from the same natal nebula).

Horsehead Nebula

The Horsehead nebula is a dark nebula that looks like a horse’s head! It is part of the Orion Molecular Cloud complex, and has the more correct, if boring, name Barnard 33 (being object number 33 in a catalog of dark nebulae, by Barnard).

It is about 1500 light-years away, and is itself dark because of the dust of which it’s made (it’s also made up of gas, in fact it’s mostly gas, but the gas is essentially transparent). What makes it so obvious is the diffuse glow from behind it; the glow is red – due to the Balmer Hα line, a prominent atomic transition in hydrogen – and is powered by the UV light from the nearby star, Sigma Orionis (which is actually a five-star system), which ionizes the hydrogen gas in this part of the Orion Complex.

The first record of its shape is from 1888, by Williamina Fleming, who noticed it on a photographic plate taken at the Harvard College Observatory (Fleming made significant contributions to astronomy, including cataloguing many of the stars in the famous Henry Draper Catalogue). The Horsehead nebula is a favorite of amateur astronomers, especially astrophotographers (it’s quite difficult to spot visually).

The Horsehead nebula is similar to the Pillars of Creation (in M16), though perhaps not as dense; one day it too will be eroded by the intense UV from the young stars in its vicinity, and from new-born stars formed within it (the bright area at the top left is light from just such a star).

In 2001, the Hubble Space Telescope Institute asked the public to vote for an astronomical target for the Hubble Space Telescope to observe, a sort of Universe Idol contest … the Horsehead nebula was the clear winner! Hands up all of you who have, or have had, the Hubble’s image of the Horsehead as your wallpaper, or perhaps the VLT one

Universe Today has, among its stories, some good background on the Horsehead; for example Dark Knight Ahead – B33 by Gordon Haynes, Astrophoto: The Horsehead Nebula by Filippo Ciferri, and What’s Up This Week – Jan 3 – Jan 9, 2005.

The Astronomy Cast episode Nebulae explains the role of dark nebulae, such as the Horsehead, in starbirth; well worth a listen.

Sources: NASA APOD, Wikipedia


Betelgeuse is the ninth brightest star in the sky, and the second brightest in the constellation of Orion (it’s the red one, on the opposite side of the Belt from Rigel, which is the blue one, and the brightest).

With a mass of some 20 sols (= the mass of 20 Suns), Betelgeuse is evolving rapidly, even though it’s only a few million years old. It’s now a red supergiant, burning helium in a shell, and (very likely) burning carbon in another shell (closer to the nucleus), and (possibly) oxygen, silicon, and sulfur in other nested shells (like Russian dolls).

Betelgeuse is enormous … if it were where the Sun is, all four inner planets would be inside it! Because it’s so big, and is only approx 640 light-years away, Betelgeuse appears to about 1/20 of an arcsecond in size; this made it an ideal target for optical interferometry. And so it was that in 1920 Michelson and Pease used the 100″ Mt Wilson telescope, with a 20 m interferometer attached to the front, to measure Betelgeuse’s diameter.

The Hubble Space Telescope imaged Betelgeuse directly, in 1995, in the ultraviolet (see above). Why the UV? Because ground-based telescopes can’t make such observations, and because the Hubble’s resolution is greatest in the UV.

Since the 1920s Betelgeuse has been observed, from the ground, by many different optical interferometers, at many wavelengths. Its diameter varies somewhat, as does its brightness (Herschel is perhaps the first astronomer to describe its variability, in 1836). It also has ‘hotspots’, which are ginormous.

Betelgeuse is also shedding mass in giant plumes that stretch to over six times its diameter. Although these plumes will certainly cause it to ‘slim down’, they won’t be enough to stop its core turning to iron (when the silicon there is exhausted, if it hasn’t already done so). Not long afterwards, perhaps within the next thousand years or so, Betelgeuse will go supernova … making it the brightest and most spectacular supernova visible from Earth in perhaps a million years. Fortunately, because we are not looking directly down on its pole, when Betelgeuse does go bang, we won’t be fried by a gamma ray burst (GRB) which may occur (while a core collapse supernova can cause one kind of GRB, it is not yet known if all such supernovae produce GRBs; in any case, such a GRB is one of a pair of jets which rip through the poles of the dying star).

AAVSO has an excellent article on Betelgeuse, and COAST’s (Cambridge Optical Aperture Synthesis Telescope) webpage on its observations of Betelgeuse gives a good summary of one interferometric technique (and some great images too!).

Universe Today has many stories on just about every aspect of Betelgeuse, from its varying size (The Curious Case of the Shrinking Star), the bubbles it’s blowing and its plumes (Closest Ever Look at Betelgeuse Reveals its Fiery Secret), featured in What’s Up This Week, to the bow shock it creates in the interstellar medium (The Bow Shock of Betelgeuse Revealed).

Astronomy Cast’s The Life of Other Stars is a whole episode on the evolution of stars other than the Sun.


Water or Land: The Orion Landing Choice


Work is progressing on designing the new Orion Crew Exploration Vehicle (CEV), the next generation of NASA spacecraft that will take humans to the International Space Station, back to the Moon, and hopefully on to Mars. But one major question about the spacecraft has yet to be answered. On returning to Earth, will the CEV splash down in water, or land on terra firma?

NASA officials discussed various aspects of development that is currently underway for the Constellation program at a media briefing on December 10. The mobile launch platform for the Ares rocket is being built, landing parachutes have been tested and the first capsule structure of the new CEV will be constructed starting in early 2008. Design requirements for the booster rockets have been completed and just ahead are final design definitions for operational capabilities such as ground procedures at Kennedy Space Center, mission control in Houston and other areas such as spacesuit design.

Additionally research on the International Space Station has begun to help prepare for long duration spaceflights such as a measurements of microbe growth, a study of the formation of kidney stones, and a nutritional study to help understand what is “normal” for the human body in space.

But questions from the media focused mainly on the yet unmade decision of whether the CEV will land in the water or on land.

NASA originally explored multiple options for landing in both water and land. After initial studies, the first assessment by NASA and the contractor for the CEV, Lockheed Martin, was that landing on land was preferred in terms of total life cycle costs for the vehicles. But now a splashdown in water seems to be favored.

“There are a couple of aspects that pop out at us,” said Jeff Hanley, Manager for the Constellation Program. “One is the safety and the risks involved in landing. Looking at the landing itself, the event of actually touching down, water comes out to be preferable as less risk. Another aspect is the performance of the Orion vehicle as it is sent to the moon. In looking at what it takes to get a pound of spacecraft to low lunar orbit in terms of the cost, every pound that you send toward the moon is precious. From an efficiency and performance point of view, carrying 1500 lbs of landing bags to the moon and back when we have a perfectly viable mode of landing in the water near a US coastal site didn’t seem like a good trade in performance. We’ve tended toward updating our point of departure concept to now be a nominal US coastal water landing.”

The Constellation program has always considered that for the first few missions, the spacecraft would land in water until the guidance system had been tested thoroughly and proven in actual landings.

But NASA is continuing to look at landing on land as a possibility for future flights. “We want to be able to land on land in a contingency and have the crew be able to get out and walk away. Ther are limitation of what you can do on land but by the time we get done really looking at what the minimal capability of landing on land and having the crew walk away, we’ll see what the design looks like, and if the design is robust enough we could return to having nominal land landings.”

One challenge for the Constellation program has been getting the CEV light enough for the Ares rockets to be able to launch it, and therefore eliminating the 1500 lb airbags for landing has its appeal.

“The predominant design philosophy for Orion and Ares 1 has been that we are designing for lunar missions,” continued Hanley. “We will service the International Space Station within that set of capabilities. From that perspective, designing a lot of mass into the spacecraft just to enable land landings has not traded out to be an effective use of our performance. That’s the major consideration in play. Right behind that are life cycle costs.”

Making the decision of land vs. water is the goal for 2008 for the Constellation program. “We’ve studied and have cost estimates for water landings against the infrastructure costs of having multiple landing sites on land and they are comparable,” said Hanley. Right now, NASA is looking at a single target landing zone off the coast of California with one or two recovery vessels.

But they are keeping their options open for a land landing. “If the Orion team is able to come in at the preliminary design review later this next year with a concept for be able to land on land that is fairly robust but not cost a lot of mass to have to hurl to the moon and back, then it becomes an operational decision,” said Hanley.

There has been much debate about what type of landing would be best. “There’s been a lot of assumptions made that landing on land is going to be better, but there are lot of people in the technical community that do not buy into that,” said Hanley. “There’s been a lot of debate surrounding whether or not land landing truly is better from a life cycle cost perspective and there isn’t a lot of quantitative data to really pull from.”

Hanley feels there are assumptions being made but not a lot of substantive date to clarify what the right answer is. So the next steps are to get the spacecraft to a detailed preliminary design and really interrogate the water vs. land issue. That includes further developing the operational concepts , such as how long does the capsule stay in the water, and what loads does the spacecraft see from landing on water and land. Those are all questions that need to be answered in order to make a final decision on the type of landing that will be used.

Stay tuned, as 2008 should be a year of decision for many details about Constellation and the CEV.

Original News Source: NASA News Audio