Planets and planetary nebulae have very little in common, and almost nothing to do with each other. So why are planetary nebulae named after planets? It’s William Herschel’s fault! Herschel was probably the greatest astronomical observer of the 18^th century. In 1781 he became the first person that we know of to discover a planet. He discovered Uranus. The inner planets had been known since antiquity. He also was the first person to discover types of light other than visible when he discovered infrared light. He also carefully scanned the entire sky with his sister Caroline and cataloged all of the non-stellar objects that they saw. This included many planetary nebulae. Most of these were very distant.

To better understand Herschel’s reasoning, imagine what Uranus might look like through a telescope (or remember it if you are the rare person that has seen it before). Uranus is four times the diameter of Earth, but only 1/3 the diameter of Jupiter or Saturn. It is also twice as far from us as Saturn. It is bluish green and its cloud features are extremely subtle and nearly impossible to see when looking through a telescope. Given its distance and modest size, Uranus and its planetary sister Neptune just appear as tiny blue-green "dots" in the telescope. As it turns out, that’s just what most distant planetary nebulae look like through a telescope – a blue green dot! Hence the name, even though planetary nebulae and planets have almost nothing else in common.

We will look at Uranus, Neptune, and some of the closer planetary nebulae in this observing lab. Since we will be looking at some of the closer planetary nebulae, we will see that they are not just small blue-green dots. So it may be difficult to understand why they were named planetary nebulae. Just try to imagine how they might look if they were 10 or even 100 times farther away, and you might be able to appreciate the name.

Why are we looking at them together if they are unrelated?  To be honest, they are unusual and interesting objects to observe but they do not, by themselves warrant a lab exercise.  This is an excuse that I came up with to combine them and look at them anyway.  You may very well be the first person on your block to lay claim to having seen the outer planets (and some planetary nebulae for that matter).  They won't dazzle you, but I think that they are very cool.

Objectives: 1. To observe the outer planets and planetary nebulae with your own eyes.

2. To learn more about these objects.

3. To practice using our telescopes.

Requirements:

1. Sketch of Uranus and one of Neptune as seen at high power in your telescope.

2. Sketch of at least two planetary nebulae from the list below.

Directions:

1. Before Class: Look up the coordinates for the planetary nebulae from your star atlas. Find the nearest bright star from your Bright Star List (look for those stars in your star atlas to confirm that they are in fact the closest bright star). Bring the coordinates of the planetary nebulae and their attendant bright stars with you to class in an organized fashion, such as in a single table in your notebook using large numbers that are easy to read in the dark. Also leave spaces in your table for the positions of Uranus and Neptune. They will be provided to you in class. They are in the constellations Pisces and Capricornus, respectively. This knowledge will help you find the nearest bright star to them to use from your Bright Star List, so you can that part in advance.

2. Set up a telescope and align it with the north celestial pole.

3. Find and sketch, Uranus, Neptune, and at least two planetary nebulae from the list below.

Questions:

1. How did Herschel know that he had discovered a new planet when he found Uranus?

2. What did Herschel want to name Uranus? Why was it named Uranus? Who was Uranus in mythology?

3. How long does it take Uranus to orbit the Sun? How large is Uranus compared to Earth? What would Earth look like in the telescope from that distance? What does the Sun look like from Uranus?

4. How and when was Neptune discovered? What is it named after?

5. What is a planetary nebula? How far away are they, typically? What are they made of? How big are they? How are they formed?

6. Why do Uranus and Neptune look blue-green in the telescope? Why do planetary nebulae look blue-green in the telescope? (Hint: you may have to search the web to find the latter answer).

7. What did you think when you first saw Uranus or Neptune? How about M-57?