How Far Are the Stars? (part 3)

In my last post, I showed how Cepheid variables allow us measure distances to distant stars and even to other galaxies. However, there is a limit to how far we can measure using Cepheids.  To measure the distance to the farthest galaxies, it takes a standard candle much brighter than a Cepheid. It takes a supernova.

Measuring by Supernova

Tycho Brahe

In November 1572, Tycho Brahe noticed among the stars of Cassiopeia a bright object no one had seen or catalogued before.  From November 2-6, this ‘star’ which had been invisible but now outshone all other stars, rivaled Venus in brightness.  It then gradually faded from view, remaining visible to the naked eye until 1574.  In his published work, Brahe termed this object stella nova, or ‘new star’ in Latin.  From then on, astronomers called any star that suddenly brightened by many magnitudes a ‘nova.’

In the early 20th century, Walter Baade and Fritz Zwicky were studying novae that seemed much more energetic than most.  During a 1931 lecture, Zwicky coined the term ‘supernova’ for an event that releases as much energy at once as the sun does over 10 million years.  (As it turns out, Brahe had observed a supernova, not a mere nova).

White Dwarfs

We now know that novae and one type of supernova occur due to white dwarfs accreting matter onto their surfaces.  All stars roughly as massive as our sun end up as white dwarfs.  In a white dwarf, the nuclear fusion which normally powers the star and resists gravitational collapse has ceased.  Yet, the white dwarf fails to collapse completely because of its extreme density; a white dwarf has approximately the sun’s mass in approximately the Earth’s volume.  Thus, the matter is so dense that any further compression would force multiple electrons into the same quantum state–which is disallowed according to the Pauli exclusion principle.  The resulting resistance to further compression becomes a force known as electron degeneracy pressure.

Occasionally, a white dwarf gravitationally bound to another star gains mass from that star.  Every star has a Roche lobe, which is the region of space in which all orbiting material remains bound to the star.  If, during stellar evolution, the companion of a white dwarf expands beyond its Roche lobe, some of its material, having escaped its gravity, can fall onto the white dwarf.  On the white dwarf’s surface, in-falling material, mostly hydrogen and helium, can attain temperatures and pressures sufficient to start nuclear fusion.  The resulting fusion of hydrogen into helium and helium into carbon and oxygen releases so much energy that the white dwarf suddenly becomes up to 50,000 times more luminous. The white dwarf has gone nova.  As dramatic as a nova explosion is, however, the material ejected is typically only about 1/10,000 of the sun’s mass–much less than the mass of the white dwarf.  Therefore, a particular star can go nova on many different occasions.

The Crab Nebula is a pulsar wind nebula associated with the 1054 supernova,
recorded by both Chinese and Islamic Astronomers.

There is a limit as to how much mass electron degeneracy pressure can support.  A white dwarf must have a mass less than 1.38 solar masses (the Chandrasekhar limit) to remain stable.  If a white dwarf in the scenario described above accretes enough mass to surpass that limit, the entire star becomes unstable, resulting in a explosion called a type Ia supernova.  Unlike a nova, a supernova cannot recur because the star has been destroyed.

Supernova Types

‘Type I’ refers to scheme established by Rudolph Minkowski and Fritz Zwicky which classifies supernovae based on their spectra.  Spectra of type II supernovae indicate the presence of hydrogen.  These are supernovae in which a star much more massive than our sun collapses and explodes, skipping a white dwarf phase altogether.  Type Ib and Ic supernovae occur when very massive stars which have lost hydrogen explode.  They lack hydrogen in their spectra, which puts them in type I, but they are more similar to type II supernovae in how they form.  Type Ia supernovae, in which white dwarfs explode, are the most interesting for our purposes here.

Standard Candles

In every type Ia supernova, then, the same amount of mass (about 1.38 solar masses) is exploding.  As a result, each type Ia supernova has the same intrinsic brightness, or luminosity.  Recognizing this, Walter Baade proposed using them as standard candles back in 1938.  Further, such a supernova is brilliant, rivaling the brightness of an entire galaxy, and therefore visible over much longer distances than Cepheids.  For these two reasons, type Ia supernovae are excellent standard candles for measuring distances to galaxies containing them.  With type Ia supernovae, we can measure distances many hundreds of millions of parsecs away, as compared to only about 29 million parsecs for Cepheids.

Champagne Supernova

There are two caveats, however.  First, astronomers using the Mauna Kea Observatory in Hawaii in 2003 observed an atypical type Ia supernova, which they dubbed the ‘Champagne Supernova’ (nicknamed after the Oasis song of 1996).  Somehow, this white dwarf managed two solar masses before exploding (rather than 1.38).  Some suspect that an unusually fast rotation may have allowed the extra mass to accrete, but this is an area of ongoing research–a reminder that whenever we think we have something figured out, nature can surprise us.

Second, although measruing galaxies hundreds of millions of parsecs away is a great achievement, we estimate that the observable universe is 28 billion parsecs across.  There are still other tools we must use to measure even more distant wonders.

Can’t see the video? Check out this video on Supernovas by clicking here.

Earth Day at HMNS April 30, 2011

Earth Day 2007 - Atlantic Reflection
Creative Commons License photo credit: FlyingSinger

April begins the Spring season.  The sun lasts longer, flowers start to bloom, and romance can be in the air.  April was originally the second month of the Roman calendar and is probably named for the Latin word aperire, meaning “to open.”  It is always a wonderful thing to see animals born during the winter come out, to have days where the daytime requires shorts and a t-shirt, but the nights need a jacket. April is also Jazz Appreciation Month.  Some nice lively music for a lively month (if you’re in Houston check out the Discovery Green on Thursdays). The gem for April is a diamond and the flower the daisy.  Both are bright and shiny.

So what else happens in April?

One of my favorite holidays of the year is on April 1.  Although in recent years I have tried to tone down my pranks after the incident with the fencing equipment room and the new lock.   Some of the other celebrations that passed this month are just for fun (like Caramel Popcorn Day on April 7 or Scrabble Day on April 13).  Other celebrations are more serious such as the celebration of the first man to leave the atmosphere (Yuri Gagarin on April 12, 1961) or the sinking of the unsinkable boat (RMS Titanic on April 15, 1912).

The big thing in April for the Wiess Energy Hall and the ECC is Earth Day.

Earth Day was on April 22 and is a celebration about conservation.

Senator Gaylord Nelson

The first Earth Day was on April 22, 1970.  It was created by Senator Gaylord Nelson from Wisconsin.  He wanted to generate political concern for the environmental movement.  While there was another Earth Day proposed to President Kennedy, Senator Nelson decided to have a decentralized, bottom up approach.  Instead of having Congress enact something on a national scale, the senator wanted the events to come from local interests.  He also thought of it as a teach-in (for those of us who are not from the 1970’s a teach-in is where a bunch of people, usually students and teachers, get together and talk about a broad issue.  It’s like a forum, but with specifics.  Silly ‘70s).

On the first Earth Day, over 20 million Americans participated, mostly on college and university campuses.  April 22 was chosen because it was between Spring Break and exams (and had nothing to do with Vladimir Lenin).

Since then, Earth Day has evolved into a global celebration.  In 1990, the 20 Earth Day, 200 million people celebrated in 141 countries.  With the invention of the internet, more and more organizations have been able to get involved.  In 2009 the United Nations recognized April 22 as International Mother Earth Day.  This year the tag line is “A Billion Acts of Green.”

On April 30, the Houston Museum of Natural Science will have its big Earth Day celebration from 10-2 (and it’s free). I’ll be helping to make paper wind turbines.  Drop by and join me.

Flickr Photo of the Month: Texas Skies! [April 2011]

George Observatory at Brazos Bend State Park
George Observatory at Brazos Bend State Park by Andrew Fritz.
View Large.
Posted here with permission.

There are some amazing photographers that wander the halls of HMNS – and our satellites southwest of Houston. When we’re lucky, they share what they capture in our HMNS Flickr pool. Each month, we share one of these photos here on the blog.

As the weather gets warmer, you may be starting to think about how to get outdoors to enjoy this brief period before our infamous Houston summer gets well and truly unbearable. Our George Observatory is a little-known but not-to-be-missed experience – nestled in Brazos Bend State Park, far enough from the city lights that you can still see the wonders of the cosmos with your own two eyes. And beyond that – you can get a closer view of planets like Saturn (remarkably radiant right now) through the Observatory’s Gueymard Telescope – one of the largest in the country that is available for public viewing.

I loved this photo of the George Observatory by Andrew Fritz, who generously shared it in our Flickr pool and also agreed to let us share it with you. Here are his thoughts on the image:

I live in Richmond (far SW Houston Area) and so Brazos Bend State Park (including the George Observatory) is in my back yard and is my local photographic stomping ground. It is one of the hidden gems of the Houston area and I frequently visit it around sunset when the bird life and light are amazing.

On the night I captured this image I had recently received some new camera gear and this was my first chance to try star photography with it. I hung around in the COLD waiting for the sun to finish setting. Once it had, I spent several hours (yes, hours) taking a few shots around the observatory. An observatory in front of stars is a classic shot. When you are new at something, try what you know works first.

Technical Details:
*Camera: Nikon D700
*Lens: Nikon 28-70mm f2.8 @ 28mm and f2.8
*ISO Speed: 1600
*Shutter: 10s
*The shot was captured on a tripod.
*The dome of the observatory was lit with a small LED light.
*The glow in the sky is Houston.
*The image was shot in RAW and processed using UFRaw.

Visit our George Observatory any Saturday night! When the weather is clear, there’s no better view of the universe.

You can see more of Andrew’s photography on his blog.

Inspired? Most of the Museum’s permanent galleries are open for photography, and we’d love for you to share your shots with us on Flickr, Facebook or Twitter. Check out the HMNS photo policy for guidelines.

Get your hands on science!

Terrence McGovern, a volunteer here at HMNS, does several chemistry shows a week to help teach the basics to our visitors. His show, geared towards both kids and adults, is a great way to see science in action.

Terrence explains the principles of density, acids and bases, and polymers in a way that makes it easy to understand no matter what your age is or your background in chemistry. A simple experiment with an egg, saltwater and freshwater helps to show how density affects whether or not items will float.

 Terrence McGovern demonstrates how density works
with the help of a member of the audience.

Terrence also shows how acid and bases can react with one another. Filling a bottle with vinegar (an acid) and placing a little baking soda (a base) in a balloon, he shows how the two react to form a gas that inflates the balloon.

 
 Chemical Reaction!

So come on down to HMNS and see one of Terrence’s many shows and learn all about chemistry.

This show is free for patrons with a ticket for our exhibition halls. For more information and show times, call the box office at 713-639-4629.