Ultraviolet Telescopes: Looking at Hot Stuff

Ultraviolet Telescopes: Looking at Hot Stuff

In the earlier article Infrared Telescopes, we showed that Infrared was for looking at some of the oldest and coldest structures in the universe—those with very low energy levels.  These would include nebulae, extremely tenuous filaments that join some galaxies together, or the branes that form “sheets” of galaxies that comprise the Universe, with vast empty spaces in between, much like this image. 

At the other end of the spectrum (no pun intended) is Ultraviolet (UV), which is remarkably useful for looking at very hot objects like young, bright stars, or elderly stars, producing prodigious amounts of energy prior to going nova or supernova, and nearing the end of their (cosmic scale) lives.  By using spectroscopic analysis, we can look at a star’s UV light emissions and see what elements are in its atmosphere.  We can determine its age and figure out its evolutionary history.  UV is very important in stellar studies.

In this dual image, we can see a galaxy named M81.  The name comes from simply being the 81st object in the 110 object Messier Catalog.  This was a listing of deep-sky "annoying stellar objects," which frustrated astronomer Charles Messier in the 1700s while conducting his search for comets.  Even less distant objects, like the Andromeda Galaxy (aka M31, only 2.5 million lightyears away), often resembled the fuzziness of comets before we had more powerful telescopes.

M81 is ~12 million lightyears away.  The upper image was taken by the Hubble Space Telescope in the visible light spectrum and is so incredibly detailed that individual stars are visible.  You can look at the original high-res image here, but bear in mind that the file is very large (22,620 × 15,200 pixels), which makes it at least 343 MB, bigger than is useful for most portable devices. 

The point, however, is that the lower portion of the image produced by GALEX (Galaxy Evolution Explorer, a since decommissioned orbiting UV space telescope) is still M81, but produced in UV light.  Now it only shows the hottest sources, and the cold matter is cast aside (or, more specifically, cool material lacks the energy to register, so the image seems clear of all the ordinary visible light and Near Infrared that is typically recorded).  It is so stark that it almost looks hand-drawn.  A more detailed version is available here if you want to look at these hot, young bodies.

Nearly Impossible on the Ground, but…

          Several telescopes have served us in the Ultraviolet range.  Virtually all of them have been in space, and some others have been mounted on high-flying atmospheric balloons or high-altitude rockets.

Nearly all of them would have been unlikely to exist without the efforts of African-American scientist George Robert Carruthers, who invented the Far-Ultraviolet Camera Spectrograph in the early 1970s.  It was specifically designed to suit the needs of the Apollo 16 mission to the Moon but evolved from his work going back to 1964 when he first started working for the U.S. NRL (Naval Research Laboratory).

Of course, as with all space scientists, he was very well qualified.  He obtained his Bachelors in Aeronautical Engineering in 1961 at age 22, his Masters in Nuclear Engineering in 1962, and his Ph.D. (Aeronautical Engineering) in 1964.  The fact that he is a talented inventor was a real bonus, and that skill earned another of his cameras space on a Shuttle flight.

Why Space?

          By the time you get to the top of Mount Everest at 8.5 km (5¼ miles), there remains only 28% of the air pressure felt at sea level.  This is inadequate for humans to live, which is why so many climbers die every year in the last few hundred feet of the ascent.  The top peak is referred to by climbers as the Death Zone, or Kill Zone, for this reason.

          Nevertheless, it is still too thick for UV to penetrate successfully.  The majority of the Ozone layer exists between 15 and 40 kilometers (10-25 miles) high.  Ozone exists there because the UV light arriving from space expends itself in that sufficiently dense region. 

High-energy UV crashes into oxygen atoms, breaks them apart from normal molecular O2, to make monoatomic O1, or simply O, that then bonds to an O2, to form O3, or as we more commonly know it, Ozone.

          It a fascinating process, but the important thing to note is that all but a tiny fraction of UV is stopped by our atmosphere.  If you want to look at the Sun in UV, earthbound observations will be fairly useless.

          In this shot, captured in 2015, the Sun (top image, in visible light), looks calm and quiescent with just a few tiny sunspots.  In UV (lower image, taken at the same time), you can see a huge “cold” area in the NE, but many, many hotspots around the equatorial region. 

When you compare it to the "visible" Sun, suddenly, you can spot all the tiny, almost invisible sunspots shown as “hot areas” of the UV image.  The ultraviolet renders what you might have overlooked as plainly visible.

Different Bands

UV runs from 10 nm to 400 nm, sitting just above visible light, and just below X-rays.  Most of the energetic UV rays are filtered out by our atmosphere, and what remains can cause suntans or skin cancer.  Our body responds by producing melanin to protect us from that, but also uses the energy of UV to produce vitamin D to make our bones stronger.

What’s Up There

There have been numerous UV space telescopes and observatories that utilize UV as part of their mission.  These include:

  • The Solar Dynamics Observatory (SDO), operating since 2010, monitors the Sun and its activity over several different frequency ranges from 9.4 nanometers up to 170 nm, plus white light (~450 nm);
  • The Solar and Heliospheric Observatory (SOHO), operating since 1995, was also designated to study the Sun’s atmosphere, helioseismology, and space weather. It has (incidentally) allowed the discovery of more than 3,000 new comets, aiding us in calculating their orbits and potential threats to our planet;
  • The Geostationary Operational Environmental Satellite system (GOES), is a string of continuously launched satellites used for Earthly weather reporting. They feature UV and extreme-UV observing capabilities as well as visible-light imaging capabilities to track large storm systems;
  • The Hubble Space Telescope (HST) is still running, having its 30th operational birthday on Jan/2/2020, and its original 15-year mission, already exceeded, might be extended to run until 2040. It observes all across the spectrum from Infrared to X-ray;
  • FUSE or the Far Ultraviolet Spectroscopic Explorer observed in the 90.5–119.5 nm band. It launched in 1999 and lasted until 2007 when its pointing mechanism failed.  Fuse made it possible to understand deuterium distribution and study galactic chemistry and chemical evolution, locally, immediately outside the Milky Way, and intergalactically as well.

The Takeaway

          UV shows us the hotspots in the universe that our eyes cannot see unaided.  It clears away the visible light that can obscure subtle elements, giving us a way to focus more tightly on hidden details. 

Beyond UV telescopes lay the X-ray and Gamma-ray telescopes that reveal even finer details.  In the other direction, below infrared, lay the microwave and the radio wave telescopes that reveal even more of our hidden Universe, and we’ll look at those next!

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