Week of Moons: Moon Facts

Today I thought I'd give some interesting facts about our beloved moon.

 

The moon...

• Spins at a rate of ~10mph
• orbits earth in about 27.3 days
• Is not made of cheese (as apparently 13% of the population still believed in a survey conducted in 1988). It's actually made of heavy metals, mostly iron and silica. The surface is covered in a dusty glass like material called silicon dioxide.
• Has a core which is partially molten.
• Surface consists of dust that smells like burnt gunpowder
• Is tidally locked to Earth which means the same side of the moon is always facing Earth viewers
• Is covered in craters because it has no atmosphere to destroy objects before they crash into the surface
• Is about 1/4 the size of earth and has a surface area of about 9.4 billion acres
• Is the only other astronomical body that humans have set foot on.
• Has small amounts of water ice near its surface
• Causes tides here on Earth

Image Credit: Luc Viatour  www.lucnix.be

Week of Moons: An Overview

This coming weekend I'm helping with a moon activity for ASRAS' Young Astronomers Program, so I thought I'd talk a bit about moons in our solar system this week.

We've all looked up at night and seen our big, bright, shiny moon glaring back at us. The moon actually doesn't shine at all. It only looks bright because it's reflecting light from the Sun! Earth is not the only planet with a moon. Most of the  other planets in our solar system have moons, and some even have multiple moons! A moon (or natural satellite as astronomers often call the moons of other planets) is loosely defined as an object that is  gravitationally bound to and orbiting a planet. Below is a list of each planet and the number of moons it has.

Mercury: 0

Venus: 0

Earth: 1

Mars: 2

Jupiter: 63

Saturn: 47

Uranus: 27

Neptune: 13

Moons come in all shapes and sizes, and most don't look anything like our moon! They don't have to be round white objects made of sand and rock. Some moons are made of metal while others some sort of liquid. Jupiter's moon Io even has erupting volcanoes on it! The picture below shows some of the more popular and well known moons in our solar system to scale so you can see how other planets moons size up to ours!

(Phobos, Dactyl and Deimos are there, they are just very tiny in comparison!)

Image credit: NASA

Light Curves!

We've already learned about the Kepler Space telescope and how it searches for planets. The basic idea, just to refresh your memory, is to watch a star for a long period of time and see if it changes in brightness periodically. If it does, then the dimming of the star might be due to a planet crossing in front of, or transiting, the star and blocking some of its light! When astronomers do this sort of thing they construct what's called a "light curve". A light curve is just a graph with star brightness on the vertical axis and time on the horizontal axis. Each time you measure the star's brightness you put a dot on the graph corresponding to the time you took the measurement and how bright the star is. A real example of some light curves constructed from Kepler data are shown below.

 The star appears to be at normal brightness, or 1 on the graph, most of the time. But once in a while we see a tiny drop in brightness (less than 0.1%). This drop in brightness occurs once every few days and lasts for a few hours. Not only can light curves reveal the existence of a planet, but they can also tell us the size of the planet, how far away it is from its host star, and where it sits in orbit! As you can see from the cartoon in the image, smaller planets block out less light, so we see a smaller dip in the light curve. Also, planets that cross in front of the star near the top or bottom rather than the middle have a thinner dip in the light curve. If we know how big the star is (and often we do) and we know the time between light curve dips, we can calculate the period of the planet (i.e. how long it takes for the planet to revolve around the host star one time) and from that get the distance between the star and the planet.

The coolest thing about light curves and planet hunting is that you don't need a big, fancy  and expensive telescope to do it. The very first discovery of an exoplanet was made using a digital camera and a 4inch wide backyard telescope! With just basic equipment and some astronomy knowledge you can make light curves and hunt for planets right in your own backyard.

Image Credit: NASA/Kepler

Direct Imaging of Exoplanets

It's not easy to take pictures of exoplanets for two reasons:

1) They are small and far away

2)They are very dim compared to the star they revolve around

Imagine you're standing in the end zone of a football field at night. Taking a picture of an exoplanet would be like trying to take a picture of a small moth fluttering in front of a stadium light at the other end of the field. The stadium lights are way too bright, and the moth is too small and far away to make out any details. The most common method used to "image" exoplanets is to look at light curves, which I will discuss on Thursday's post, but astronomers have actually discovered a way to take pictures of some exoplanet systems. To do this they use an instrument called a Vortex Coronagraph. This device was actually experimentally discovered by Grover Swartzlander, a physics/imaging science professor at the Rochester Institute of Technology! Simply put, the device "blocks" all the light from a star, allowing any planets around the star to appear very bright. It's a bit more complicated than it sounds, as the device doesn't really block the light, it uses mathematical tricks to polarize the light and cause the light waves to cancel each other out. 

The picture above is of the exoplanet system around star HR8799 taken using a Vortex Coronagraph on the Hale 1.5m telescope. The star is where the green X is, and as you can see, it's light has been canceled out so that we can see three planets. Only a few exoplanets have been imaged this way, but as more discoveries are made and technology advances, we hope to directly image as many systems as possible.

The Universe Needs A Speeding Ticket!

One topic astronomers don't talk about very much is how fast things move in outer space.  I'm sure most of you reading this have either driven a car or taken a ride on a roller coaster and felt how fast ~60mph is. You're hair's blown back, your face forced into a smile, trying not to scream as your flying down the roller coaster hill. It's really fast from a human perspective! Yet believe it or not, you're actually moving much faster than that all the time, and you don't even realize it! Everything in the universe is moving, including Earth, the Sun and the Milky Way. Below is a list of speeds (in miles per hour) that these objects are moving at, just to put things into perspective….

• The Earth spins on it's axis at a speed of 1,040 mph
• The Earth orbits around the Sun at 66,615 mph
• The Sun and our solar system orbit the center of the Milky Way at 447,387 mph
• Our Milky Way galaxy and the Andromeda galaxy are hurling towards each other at 310,685 mph
• The universe is expanding at a rate of ~180,000 mph

And you thought airplanes and rocket ships moved fast! Now do you agree with me that the universe needs a speeding ticket?!

Why Is Space Black?

I think everyone at some point has looked up and asked "why is the sky blue?" , and if you go to the week of 3/27/11 in the archive you can find the answer! But have you ever looked up at the night sky and wondered why is the sky black? Well duh! It's because your not facing the sun, which lights up the sky during the day, right? Well, not exactly….

Think about this. The Universe is huge! Our galaxy is filled with hundreds of billions of bright stars, and the universe is filled with billions of bright galaxies. In every direction we look, there are bright sources of light, and this light has been traveling towards earth for over 14 billion years. So we would actually expect the night sky to be beaming with light from stars and galaxies! But obviously it's not, so what's going on? Any guesses?

Ok I'll tell you. The fact that the night sky is black is proof that our universe is expanding!  What's happening is that as the universe expands, objects move away from us. The light emitted by these objects is Doppler shifted, which means it's shifted towards the "Red" part of the electromagnetic spectrum. The faster it moves, and the farther away the object is, the more red the object looks. Many of objects in the night sky are either so far away that they are too faint to be seen with the naked eye, or their light has been red-shifted into a part of the spectrum that our eyes are not sensitive to!  And that is why we don't need "star-glasses" at night!

 Image Credit: DeviantArt: ~andrei030

The Electromagnetic Spectrum (aka light)

Astronomers use electromagnetic radiation (fancy word for light) to study astronomical objects. Light can come in many varieties. There is visible light, which we see with our eyes, but there is also other types of light that human eyes are not sensitive to. For example, when you go to the hospital to have an x-ray, your body is exposed to x-ray radiation. Night vision cameras allow you to see infrared radiation, which is essentially the heat given off by different objects. The image below shows you all the different types of light, what wavelength and frequency they have, and compares the size of the wavelengths to a common object. 

 

 

It was not long ago that astronomers realized that outer space glows in more than just visible light. With special cameras mounted on telescopes, astronomers can see how objects look at any light wavelength they choose. And boy were they surprised at the results! Below is an image of the cartwheel galaxy. On the right hand side is the galaxy in the x-ray, Ultraviolet (UV), visible, and infrared (IR) wavelengths. The big picture is a combination of all these pictures. 

  As you can see, the galaxy looks very different in each of the four different types of light. You can't really tell that the object is a galaxy in the x-ray, but in the visible and IR, three galaxies are visible! This use of the whole electromagnetic spectrum allows astronomers to study outer space in new ways and discover things that were "not visible" just 30 or so years ago!

Image credit (bottom):  Chandra, GALEX, Hubble, Spitzer - Composite: NASA/JPL/Caltech/P.Appleton et al.