Four hundred years ago Johannes Kepler (the same Kepler who gave us Kepler’s laws) saw a new bright star in the sky: a supernova. Obviously, Kepler couldn’t take a photo of it, but he did take detailed notes. Those notes have allowed us to determine where in the sky we should look to see the remains of “his” supernova. Interestingly, this remnant has been puzzling current astronomers as much as it puzzled Kepler – even with all our new instruments.

The puzzle is this: what kind of supernova was it? Supernovae come in two basic flavors: Type Ia and Type II, and scientists can’t tell which Kepler’s was.

Type Ia:

Type Ia supernovae occur when you have two stars orbiting in a binary system: a white dwarf star and a larger companion star. If they’re close enough together, gasses fall from the bigger star onto the white dwarf, and if this happens too fast, the star explodes

Want More?

http://imagine.gsfc.nasa.gov/docs/introduction/supernovae.html

Polaris, the North Star, the 49^th-brightest star in the sky, is a Cepheid variable star. Cepheid variables proved that galaxies are huge and far away instead of small and close by.

Why Stars Are Big:

A star stays star-sized the same way a balloon stays balloon-sized: competing forces. In the case of a Helium balloon the Helium is pushing out, trying to escape from the balloon. If it weren’t for the air around the balloon, it would explode – allowing the Helium to escape. The air (the atmosphere) is pushing back on the balloon, trying to flatten it.

In the case of a star, the energy from the fusion reaction at the core of the star is pushing out, keeping the star big, and gravity is pushing in, trying to collapse the star into a neutron star or black hole. As long as the fusion keeps going, the star stays “inflated.”

Pulsing:

Unfortunately, Cepheids have finished fusing all of their Hydrogen into Helium. The next step is to fuse Helium into Carbon, Nitrogen, and Oxygen – but that reaction is hotter and takes more energy to start – energy the star doesn’t have. Since there’s no longer fusion happening in the star, gravity begins to win. The star starts to collapse: squishing all the atoms in the star closer together.

But wait! Squishing releases heat and energy! Suddenly, the star has enough energy to fuse some Helium into other elements. Fusion begins. The energy from fusion pushes out on the star, and the star expands again – losing energy and heat. Sadly, there is no longer enough energy to keep fusing Helium so fusion stops. The star begins to collapse again – releasing heat, energy, and fusion! This pushes out – heat is lost – fusion stops – the star squishes – heat is gained – fusion starts – the star expands … etc, etc, etc.

What Does This Have to Do with Galaxies?

In Cepheids, the speed of the pulsing is related directly to how bright the star is (a period-luminosity relationship). So, if you know how fast a star is pulsing, you know how bright the star really is (not how bright it looks in the sky). Then, by measuring how bright the star looks you can figure out how far away it must be. It’s like a flashlight: if you hold the flashlight and shine it in your face it’s very bright; but if you have a friend hold the flashlight at the other end of a football field and shine the flashlight in your face, you can look right at it: it looks dim.

In 1912, Henrietta Leavitt measured a bunch of Cepheids in the Small Magellanic Cloud and realized that they were too far away to be within our galaxy. Astronomers realized this meant that some things they thought were small might be large and very far away: galaxies.

There are some problems with measuring the distance to galaxies this way, but it got us started.

Want More?

http://zebu.uoregon.edu/~soper/MilkyWay/cepheid.html

Where’d I Get My Info?

http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/cepheid.html

http://zebu.uoregon.edu/~soper/MilkyWay/cepheid.html

It’s cold. It’s dark. It’s wet. There’s no fresh produce. It’s clearly time for a celebration.

It’s the winter solstice. This year (2006) the solstice occurs just after midnight at 12:22 a.m. between Thursday, December 21^st and Friday, December 22^nd. You may already know that the winter solstice is the shortest day of the year, and here are some other cool facts that you may not know.

Dark Days of Winter:

The United States Naval Observatory (USNO, a reliable source for all kinds of astronomical measurements) brought the “Dark Days of Winter” to my attention. Although the winter solstice is the shortest day and the longest night of the year, it’s not the day that the sun rises latest or the day the sun sets earliest. The earliest sunset happened two weeks ago on December 5^th. Unfortunately, even after the solstice it’s going to continue to become harder to get up in the morning because the sun will be rising later and later all the way until January 3^rd.

These dark winter days are caused by the fact that the Northern hemisphere of the Earth (where we live) is tilted away from the Sun in the winter, not because the Earth is farther from the Sun. The opposite is true on January 4, 2007. At 3 p.m. the Earth will be at the point in its orbit closest to the Sun, which is known as perihelion. At midnight on July 7, 2007, during the height of summer, we will be at aphelion or the furthest point from the Sun.

Update:  An image to help you interpret how it is possible to have the earliest sunset and latest sunrises on different days than the shortest day of the year. Yes, this is confusing so think about it for a little bit. At a future date I will attempt to write a full explanation.

Approximated Diagram of the Earliest Sunset, Latest Sunrise, and the Shortest Day of the year. Attrib: Alice Enevoldsen

Interpretation Ideas:

This is easy to work into any star show this week, either when talking about the North Star or just before showing the sunrise. Whip out the Earth-on-a-stick, do a quick seasons demo, or just show the earth’s tilt. If you have more time, show how high the sun is on the meridian on the day of the winter solstice, and then again later in the year.

Want More?

http://aa.usno.navy.mil/faq/docs/dark_days.html

http://aa.usno.navy.mil/

Where’d I Get My Info?

http://aa.usno.navy.mil/faq/docs/dark_days.html

http://aa.usno.navy.mil/data/docs/EarthSeasons.html

http://aa.usno.navy.mil/data/docs/RS_OneDay.html

~ A l i c e !

“Mars Has Water!” News Releases

This table is updated often. The latest update is the date of the latest news release.

Laugh Out Loud Cats - Mars Has Ice! - Adam Koford

From December 8, 2006

The Mars Global Surveyor, a satellite that has been in orbit around Mars since 1997, sent back a picture containing evidence that water flowed on Mars sometime during the last five years. These pictures were released by NASA on December 6, 2006, though the photos were taken in 2004 and 2005.

The evidence shows new deposits within a crater that look like they were left by flowing water. Simple enough evidence, so let me answer a few specific questions about this story.

How come we’re just hearing about this now?

As of November 2006 the Mars Global Surveyor stopped responding to NASA Control (it had a lifetime of 9 years – 7 years longer than expected). The pictures containing the evidence were taken and sent to Earth in 2004 and 2005. As with most NASA missions, we get more data back than we can possibly sift through as fast the satellites send it to us. To tell you the truth, they’re still sorting through some of the data from the 1996 Pathfinder mission to Mars (the first rover). Expect to hear more discoveries from the Mars Global Surveyor in the future, even though the satellite itself is now defunct.

Background Knowledge: Erosion and Deposition

Okay, time for a quick geology/planetology class. Do you remember the vocabulary words sediment, erosion, and deposition from some science class you had? As water flows it picks up (erosion) bits of dirt and whatever else is on the surface (sediments). When the water slows down it drops those sediments (deposition).

Water is the best eroder we know. Wind can erode too, but not nearly as effectively, and wouldn’t leave sediments in the same patterns as water. That’s why we’re excited by these new photos: the deposits look like they were left by water.

Why is this important?

Life as we know it needs liquid water. Life is cool.

Is this news?

“Wait,” I hear you saying, “Didn’t this news come out several years ago? And then didn’t we hear it again a couple of years ago? What’s with this?”

You’re right: this is the fourth (or maybe fifth) reliable announcement in recent history that “Mars has water.” The difference between all these announcements is the strength of the evidence, the amount of water found, and the estimated date that the water was liquid.

In the 1970s, we observed the light bouncing off Mars’s ice caps, and determined that they contained at least some water, though they were mostly frozen carbon dioxide (dry ice). Two decades later, in 1999, we were able to look at photos of the surface of Mars that showed outlines of riverbeds, floodplains, and smooth surfaces: all evidence of “recent” water activity. In this case, “recent” means geologically speaking: within several million years. In the year 2000, scientists received pictures of gullies carved by water: gullies that could have easily been formed within the last 100 years. Or, they could have been formed a million years ago, since we haven’t observed much erosion on Mars.

More recently, in 2001 (2002?), scientists detected large amounts of underground hydrogen. Since water is H2O (two hydrogens and an oxygen), underground hydrogen indicates the possibility of large deposits of water. Other evidence tells us that this water would be frozen, if it’s actually there.

Two days ago we got new news. Between the time that we took a picture of a few certain crater walls in 1999, and the most recent pictures of the same craters from 2004 and 2005, new dirt (the fancy word for dirt on another planet is regolith) had been deposited.

Want More?

Water Precipitation on Mars

NASA’s Mars Pages

Where’d I Get My Info?

Mars Global Surveyor

Press Release

Press Release

Space Science

What is a Planet?

The International Astronomical Union (IAU) voted on the definition of planet on August 24, 2006.

A Planet is:

1. Orbiting a star.

2. Big enough to be spherical – an IAU process will decide if any new bodies discovered are planets or not.

3. Massive enough to clear its orbit of similar bodies.

A Dwarf Planet is:

4. Orbiting a star.

5. Big enough to be spherical.

6. Not massive enough to clear its orbit of similar bodies.

7. Not a moon.

In addition, other categories can be used to be more specific:

A Plutonian (a new official category, but not an official name):

8. A dwarf planet similar to Pluto in size and orbit.

A Small Solar System Body is:

9. Anything that’s not a planet.

Planets (as currently recognized by the IAU):

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

Plutonians:

Pluto, Eris

Dwarf Planets:

Ceres, Pluto, Eris

Planets that could be added under this definition:

The bodies discovered over the last few years: “Sedna,” “Quaoar,” and many other recently discovered Trans-Neptunian Objects.

The current asteroids Vesta, Pallas, Hygeia

Watch the Aurigids at 4:30am on the night between August 31-September 1, 2007 for an “outburst” of meteors lasting about two hours. (It’s called an “outburst” instead of a “shower” because it’s so short and bright).

Why should I get up at 4:30am?

Although we’re only talking about several dozen meteors, these should be some of the brightest ones you’ve ever seen. According to NASA Ames Research Center, we will be looking at meteors brighter than the brightest stars in the night sky.

This is the only predicted meteor shower from a long-period comet (a comet that passes the Sun less than every 100 years or so) in our lifetimes. This is so exciting, cool, and rare that a conglomeration of scientists is going observe the meteors from a plane flying over California during the peak of the shower.

Where are they from?

Comet Kiess circled past the sun some time in the 40 years around 4 C.E., and again in 1911. The dust particles that make the Aurigid meteors are from this comet, but they’re pushed in and out of the Earth’s orbit by the gravitational forces of the Sun and other planets, so we don’t see the meteors every year. The last sightings were in 1935, 1986, and 1994.

Want to be Official?

Take part in the campaign! You can contribute to the study of the Aurigid shower by: recording your visual observations. Count meteors in 1-minute intervals. Do not change viewing direction, keep Moon out of field of view. The Aurigid Meteor Observation Project explains how to use a laptop to do the counting for you. Report your observations to: pjenniskens [at] seti.org.

Want More?

http://aurigids.seti.org/ – More information about the mission, and how you can contribute.

http://leonid.arc.nasa.gov/aurigids.html