Field Journal from Max Mutchler - 3/1/96

HELPING POINT THE HUBBLE AT CVZ OBJECTS

Well, they are almost all done now, so I have a chance to describe what a week like this can be like.....

One of the observing programs is for a very fortunate amateur astronomer. Most of the astronomers who use HST are professional astronomers who work as college professors or researchers at science labs or institutes (they have earned the formal title "Dr."). However, once in awhile our director gives some of his personal HST time to an amateur astronomer who has submitted an interesting observing proposal that would make good use of HST's abilities. I have worked with several of these HST amateur astronomers over the years: they have observed other planets and their moons, exploding stars, gravitationally lensed objects, galaxies, and quasars.

The amateur astronomer I am currently working with will take pictures of a strange galaxy called NGC 1808 that has mysterious pillars of material spouting out from it's center. Nobody is quite sure if these "fountains" of galactic material are caused by a black hole in the center of the galaxy, or possibly by a burst of star formation near the center. Only HST can see into the center of the galaxy to see what is going on there.....I can't wait to learn the answer!

Four of the other observing programs were difficult to implement because we are planning to use special orbits where the target (a planet, stars, or a galaxy, etc...) is visible for a larger-than-usual amount of time.

HST orbits the Earth every 96 minutes. If HST is pointing at a target and orbiting the Earth, it's not hard to imagine how for about half of the orbit, the target is not visible because the Earth itself is in the way (until HST swings back around the Earth, and then the target is visible again).

Remember that even though HST is in "outer space", it is really never more than about 300 miles away from Earth (that's it's orbital altitude) -- so it's never really very far away. In fact, it's so close to the Earth that the Earth is blocking much of the view at any given moment. So we have to observe each target only when we know it will be most visible. Most HST observing programs involve many consecutive orbits. But we may only be able to see the target for about 55 or 60 minutes of the 96 minute orbit because the Earth is in the way during the other 40 or so minutes.

However, if you imagine pointing at a target that is more towards the north or south, you can see that even though HST is spinning around the Earth, that target can be visible during the whole orbit (the Earth never gets in the way). We call these areas the "Continuous Viewing Zone", or "CVZ" for short.

You can see what I mean if you hold a globe right in front of your nose. Then look at some small object anywhere in the room (like a light switch or something on a table). Then start slowly moving around the globe (but stay close to it, and keep staring at the small object!). You'll notice that at some point you can't see the object because the globe is in the way. If you continue "orbiting" the globe, it will become visible again. Now try the same experiment again, but look at a spot on the ceiling. You'll notice that as you orbit the Earth, the spot on the ceiling is always visible -- the Earth never gets in the way! Most of the spots on the ceiling (or the floor) are in your CVZ.

We try to observe targets in the CVZ as much as possible because then we have almost twice as much time in each orbit to take pictures, etc.

There are many objects in HST's continuous viewing zones, including some very important objects that are studied very often by many astronomers, because they hold special clues about how our galaxy formed (and perhaps how other galaxies form).

One such CVZ object is a globular cluster called 47 Tuc. (pronounced "47 Tuck", which is just a name given to it in a catalog: object number 47 in the constellation Tucana, which means "Toucan" in Latin (a bird with an interesting beak.... you know about this is you eat Fruit Loops cereal for breakfast!). 47 Tuc is a collection of millions of stars that all formed together billions of years ago (probably around the same time that our Milky Way galaxy was forming), and remain clustered together today because they all exert a gravitational pull on each other, which holds in each star in the cluster. This cluster is in orbit around the center of our galaxy (as are all the stars in the Galaxy, including our Sun), so we can study individual stars in it try to learn exactly how old and far away the cluster is.

Another important CVZ target is the Large Magellanic Cloud, or "LMC" for short. This is a small galaxy that is in orbit around our much larger home galaxy (the Milky Way). People who live in the southern hemisphere can see it at night -- it looks like a fuzzy white patch in the night sky (that's obviously how it got it's name, but it is certainly NOT a cloud!). Astronomers are very interested in the LMC for many reasons: it is a nearby (and easy to observe) example of the type of small irregularly shaped galaxies that are seen all over the universe. It may also hold clues about how our Milky Way galaxy formed and got so big: did a bunch of smaller galaxies like the LMC collide to form the bigger galaxies? Will the LMC crash into our galaxy someday?

I have worked on several programs to observe these targets, and since they all use the CVZ, this means they must all occur at the same time. So that's why I had to work on so many programs all at once this week. It usually involves extra work to design them to use CVZ, but as you can see, it is well worth the effort.

Now I'm looking forward to going home and playing with my 5-month-old daughter all weekend.........she's a blast! I also have to finish fixing my roof because it has been leaking since the "blizzard of the century" that hit the east coast in January. I grew up in Wisconsin so I am used to big blizzards.... this last one was so big that it made me homesick!