All posts for the month September, 2003

Web surfing can bring you some interesting astronomy images. You can’t go wrong with a link to Astronomy Picture of the Day — one of the coolest astro pics sites on the World Wide Web. I’ve been meaning to mention this site for a while — their archives contain the best and most striking images submitted by astronomers at every level. Check it out!

Remember Comet Halley — the famous “hairy star” that has been observed with great regularity once every 76 years? Which was visited by an international spacecraft armada when it last passed through the inner solar system in 1986? And which put on a fine display in the sky at that time?

Now, 17 years after that passage, this cosmic traveller has been observed at the European Southern Observatory. Moving outward along its elongated orbit into the deep-freeze outer regions of the solar system, it is now almost as far away as Neptune, currently the most distant giant planet in our system. At 4,200 million km from the Sun, Comet Halley has now completed four-fifths of its travel towards the most distant point of its orbit. It will reach that turning point — called its aphelion — in December 2023. Then it will be headed on the long return trip back through the inner solar system in 2062.

The new image of Halley was taken with the Very Large Telescope (VLT) at Paranal (Chile). It was obtained as a byproduct of an observing program aimed at studying the population of icy bodies at the rim of the solar system. The image shows the raven-black, 10-km cometary nucleus of ice and dust as an unresolved faint point of light, without any signs of activity.

The brightness of the comet was measured as visual magnitude V = 28.2, or nearly 1000 million times fainter than the faintest objects that can be perceived in a dark sky with the unaided eye.

The pitch black nucleus of Halley reflects about four percent of the sunlight falling on it, which makes it a very “dirty” snowball indeed. We know from the images obtained by the ESA Giotto spacecraft in 1986 that Halley is avocado-shaped and on the average measures about 10 km across. The VLT observation is therefore equivalent to seeing a 5-cm piece of coal at a distance of 20,500 kilometers during evening twilight. This is because at the large distance of Comet Halley, the infalling sunlight is 800 times fainter than here on Earth.

The measured brightness of the cometary image perfectly matches that expected for the nucleus alone, taking into account the distance, the solar illumination and the reflectivity of the surface. This shows that all cometary activity has now ceased. The nucleus is now an inert ball of ice and dust, and is likely to remain so until it again returns to the solar neighbourhood, more than half a century from now.

At 28.06 AU heliocentric distance (1 AU = 149,600,000 km – the mean distance between the Earth and the Sun), this is by far the most distant observation ever made of a comet. It is also the faintest comet ever detected (by a factor of about 5); the previous record, magnitude 26.5, was co-held by comet Halley at 18.8 AU (with the ESO New Technology Telescope in 1994) and Comet Sanguin at 8.5 AU (with the Keck II telescope in 1997).

Interestingly, when Comet Halley reaches its largest distance from the Sun in December 2023, about 35 AU, it will only be 2.5 times fainter than it is now. The comet would still have been detected within the present exposure time. This means that with the VLT, for the first time in the long history of this comet, the astronomers now possess the means to observe it at any point in its 76-year orbit!

This entry is based on a press release sent out from the European Southern Observatory. If you’re interested in more information about this observation, point your browser to: Comet Halley release.

Galaxy Beauty

NGC 3370

I like galaxies — especially spirals. They look spectacular and whenever an image like this one comes down from HST, I always wonder about other planets around other stars in distant galaxies. Recently this one crossed my desk. At first glance you can appreciate the spiral structure and the many stars that make up this city of light. But, there’s more to the story. Distant galaxies contain stars that we can use to determine distances. How does it work? Here’s the press release to explain:

Amid a backdrop of far-off galaxies, the majestic dusty spiral, NGC 3370, looms in the foreground in this NASA Hubble Space Telescope image. Recent observations taken with the Advanced Camera for Surveys show intricate spiral arm structure spotted with hot areas of new star formation. But this galaxy is more than just a pretty face. Nearly 10 years earlier NGC 3370, in the constellation Leo, hosted a bright exploding star.

In November 1994, the light of a supernova in nearby NGC 3370 reached Earth. This stellar outburst briefly outshone all of the tens of billions of other stars in its galaxy. Although supernovae are common, with one exploding every few seconds somewhere in the universe, this one was special. Designated SN 1994ae, this supernova was one of the nearest and best observed supernovae since the advent of modern, digital detectors. It resides 98 million light-years (30 megaparsecs) from Earth. The supernova was also a member of a special subclass of supernovae, the type Ia, the best tool astronomers have to chart the growth rate of the expanding universe.

Recently, astronomers have compared nearby type Ia supernovae to more distant ones, determining that the universe is now accelerating in its expansion and is filled with mysterious “dark energy.” Such measurements are akin to measuring the size of your room by stepping it off with your feet. However, a careful measurement of the length of your foot (to convert your measurements into inches or centimeters) is still needed to know the true size of your room. Similarly, astronomers must calibrate the true brightness of type Ia supernovae to measure the true size and expansion rate of the universe.

The very nearest type Ia supernovae, such as SN 1994ae, can be used to calibrate distance measurements in the universe, because other, fainter stars of known brightness can be observed in the same galaxy. These stellar “standard candles” are the Cepheid variable stars, which vary regularly in brightness with periods that are directly related to their intrinsic brightness, and thus allow the distance to the galaxy—and the supernova—to be determined directly. However, only the Hubble Space Telescope, equipped with its new Advanced Camera for Surveys, has the capability to resolve these individual Cepheids.

Adam Riess, an astronomer at Space Telescope Science Institute in Baltimore, Md., observed NGC 3370 a dozen times over the course of a month and has seen many Cepheid variables. Already he and his colleagues can see that these Cepheids are the most distant yet observed with Hubble. Because of their need to observe this galaxy with great frequency to record the variation of the Cepheids, the total exposure time for this galaxy is extremely long (about one full day), and the combined image provides one of the deepest views taken by Hubble. As a result, thousands of distant galaxies in the background are easily discernable.

Dr. Riess imaged NGC 3370 with Hubble in early 2003. His science only required looking at NGC 3370 in two filters that covered the visual and infrared portions of the spectrum. By teaming up with the Hubble Heritage Project, a third blue filter was added to the data to produce the composite three-color image that is shown.

Credit: NASA, The Hubble Heritage Team and A. Riess (STScI)