How Far are the Stars? (part 2)

A few months ago, I shared with you how astronomers measure distances to the nearest stars using simple geometry. I also pointed out, however, that we can measure only our small neighborhood using the geometric method we call parallax.

How then, can we possibly know the distances of stars even farther out?

Well, we all know that a light gets dimmer the farther away it is.  Therefore, we can estimate a star’s distance if we can measure how bright it appears to us and then compare that to how bright it’s ‘supposed to be.’


Astronomers describe the brightness of any celestial object as its magnitude.  The term goes back to antiquity when the Greek astronomer Hipparchus put stars into six classes of brightness.  The brightest stars he could see were called first magnitude, and the dimmest stars he could barely make out were sixth magnitude.  For one thing, this means that lower magnitudes describe brighter objects, while higher magnitudes describe dimmer objects–the reverse of what most people would expect.  Also, this means that the scale is logarithmic rather than linear, as the human eye does not detect brightness linearly.

A star’s brightness as it appears to us is its apparent magnitude.  Astronomers also define a star’s absolute magnitude as the brightness it would have if it were 10 parsecs (about 32.6 light years) away.  The difference between a star’s apparent and absolute magnitudes is the distance modulus, a direct measure of the star’s distance.  A star’s absolute magnitude is related to its luminosity (the amount of light it emits).  Objects of known luminosity, enabling us to measure their distance, are called ‘standard candles.’

Standard Candles

Among the more important standard candles for determining distances are stars called ‘Cepheid variables.’  These are stars that vary in brightness over a period of several days as they pulsate (expand and shrink again).  The period over which Cepheids vary in brightness indicates their luminosity.

Cepheids are one of several types of variable stars in the instability strip of the Hertzsprung-Russell diagram.  These stars pulsate because in these stars, a layer of helium is subjected to enough heat and pressure that helium atoms lose their electrons and become ionized.  Doubly-ionized helium or He III (with both electrons gone) readily absorbs light that a normal helium atom transmits.  Therefore, He III makes stars slightly dimmer.  However, all heated gases expand and then cool as a result of the expansion.  Thus, the Cepheid pulses outward, and in the cooler environment of the expanded star, electrons recombine with helium ions.  No longer ionized, the helium no longer absorbs light, and the star brightens again.  When the star has expanded too far, its gravity causes all the stellar material to fall back towards the center of the star.  In the heated environment of the compressed star, helium atoms lose their electrons again, the star dims, and the process repeats itself.  In 1917, Arthur Stanley Eddington suggested that Cepheids were types of heat engines; Sergei A. Zhevakin in 1953 correctly identified helium as the particular gas involved.

From Pulsation to Mass, Mass to Luminosity

Since a star’s mass determines how fast and how far it will expand before collapsing under its own gravity, the period of a Cepheid’s pulsation is related to its mass.  A star’s mass, in turn, is related to its luminosity.  As a result, we when we measure how much time it takes for a Cepheid variable to brighten, get dimmer, and brighten again, we have information about its luminosity.  Comparing this to the star’s observed apparent magnitude tells us its distance.  Once enough Cepheids have been observed, it becomes possible to establish a relation that lets us measure distances to any Cepheids, even those in nearby galaxies.

In 1784, English amateur astronomer John Goodricke discovered that the star Delta Cephei varied in brightness over a period of about six days.  Since most stars known at the time to change their brightness were novae or supernovae, Delta Cephei became the prototype of a new type of variable star.  (It turns out that a few months earlier, Goodricke’s friend Edward Pigott had discovered that the star Eta Aquila varies in the same way as Delta Cephei.  Nevertheless, the name ‘Cepheid’ remains).

Lesser Magellanic Cloud
Photo courtesy of NASA

In 1908, Henrietta Swan Leavitt was studying photographic plates that Edward Charles Pickering had taken of the Magellanic Clouds when she noticed a strong relationship between Cepheids’ brightness and the log of their pulsation period.  Leavitt assumed (correctly) that the Magellanic Clouds were much, much smaller than their distance from us; all the stars she was measuring on her photographic plates were thus at about the same distance away.  Thus Leavitt’s period-luminosity relation was a way to determine the luminosity of a Cepheid independently of its distance.

Edwin Hubble & the Andromeda Galaxy

In 1924, Edwin Hubble used Leavitt’s relation to show that the Andromeda Galaxy was indeed a different galaxy and not a nebula in our own Milky Way as many believed at the time.  In 1929, Hubble and Milton Humason used distances to galaxies calculated using Cepheids to establish that more distant galaxies recede from us faster than nearby ones, thus formulating Hubble’s law.

It turns out that there are a variety of stars in the Cepheids’ instability strip.  Walter Baade in the 1940s discovered a second type of Cepheids now called W Virginis variables, after their prototype star in Virgo, the Virgin.  Less massive and dimmer on average than the classical Cepheids, these are older population II stars with fewer heavy elements.  Conflating the two types of Cepheids had introduced errors in distances to nearby galaxies.  For example, Baade’s corrections increased the known distance to the Andromeda Galaxy by a factor of four.  Still smaller and dimmer, with shorter periods of pulsation, are the RR Lyrae variables, named after their prototype star in Lyra, the Lyre.  Astronomers use RR Lyrae stars to measure distances in our own galaxy, but their dimness makes them hard to detect in other galaxies.

The use of Cepheids as standard candles continues into recent decades as well.  The Key Project of the Hubble Space Telescope was to determine the Hubble constant (the rate at which a galaxy at a given distance from us is receding from us)  by measuring the distances to 18 different galaxies using Cepheids.

With modern methods, we are able to detect Cepheid variables in galaxies up to 29 million parsecs (94.6 million light-years) away.  With Cepheids, then, we can measure much more of the universe than with parallax alone.  However, much of the observable universe is so far from us  that it still remains out of reach.  To measure even greater distances, we will need other standard candles, which we shall discuss at a later time.

Born To Be Wild 3D – Opens in Two Weeks!

I am extra excited about our upcoming IMAX film Born To Be Wild 3D – opening a week from Friday!

It’s 45 minutes of baby elephants and teeny tiny orangutans, narrated by Morgan Freeman. It’s going to be like having a baritone comfort blanket wrapped around animal eye candy – and it’s inspiring to boot:

The film “documents orphaned orangutans and elephants and the extraordinary people who rescue and raise them—saving endangered species one life at a time.”

What’s not to love?

While you’re anxiously awaiting the release date, the makers of the film have released several fascinating behind-the-scenes webisodes from the production of the film. This one is from Camp Leakey, “a legendary place…Camp Leakey has this reputation as one of the foundations of biology.”

Take a tour of the camp with Dr. Birute Mary Galdikas, who established the wild orangutan research camp and rehabilitation center at Camp Leakey 40 years ago.

Can’t see the video? Click here to view online.

These are some amazing, dedicated people doing fascinating work. Get more behind-the-scenes goodness in the other pre-release webisodes!

Behind-The-Scenes Webisodes!
Click to watch: Borneo | Coming Home to Tsavo | Camp Leakey

UPDATE: HMNS Expansion Tops Out!

Yesterday, the HMNS Expansion construction crew poured the final section of the roof slab and the columns for the parapet screen – the highest points on the building’s structure!

Topping Out! [Marach 25, 2011]
The highest point of our new building!

So today, the museum’s contractor, Linbeck, hosted a traditional topping out ceremony and, as is customary on Texas construction projects, a barbecue lunch for 250 of the construction workers and design team members who have had a hand in helping the project achieve this important milestone.

Theories of the origins and precise symbolism of the topping out tradition of hoisting an evergreen tree to the project’s apex vary, but most agree in some form or fashion that it symbolizes both growth and good luck. Linbeck hoisted a Yaupon holly tree to the top of the Expansion and adorned it with an American flag, and Texas flag, and the Pirate flag that had been flying from boom of the tower crane. It is visible from the top of the museum’s parking garage for a few weeks, so check it out!

Here are some fun facts about the construction to date (courtesy of Linbeck):

Topping Out! [March 25, 2011]
See more photos!
  • 13,000 Cubic Yards of Concrete were used on the structure, including basement/foundation.
  • Including the steel, the structure weighs about 55 million pounds – the weight of approximately 4,000 Tyrannosaurus Rexes.
  • 10 miles of Post Tension Cables were used, which covers the distance from HMNS to Hobby Airport.
  • The tallest point on the structure is 74’-0” above the ground, or about the height of 3 Tyrannosaurus Rexes standing on top of one another.
  • There are 230,000 Square Feet total (or enough room for a football field on every floor) in the new building.
  • 128,000 Total Man Hours Worked to date (equal to about 15 years).
  • Main Exhibit Floor Volume is about 1,000,000 CF or 37,000 Cubic Yards, which would hold about 5 Goodyear Blimps.
Topping Out! [March 25, 2011]
It’s a long way to the top!

See all the photos from the Expansion on Flickr | View all news on the HMNS construction to date!

The museum would like to thank Linbeck and Gensler and all firms and individuals involved in accomplishing the exciting work in place to date. Visit the HMNS web site to learn more about the exciting exhibitions coming soon to HMNS!

Kid’s Reading List! Texas Tales

To complement our new Texas! exhibition, we have created a book list for you and your kids to read. In today’s blog we talk about two books by Tomie dePaola.

Few American artists are more beloved than Tomie dePaola.   Tomie and his work have been recognized with the Caldecott Honor Award (awarded annually to the most outstanding picture book for children), the Newbery Honor Award (awarded annually to the most distinguished contribution to American literature for children) and the New Hampshire Governor’s Arts Award of Living Treasure.  And few elementary school picture books have been read by more students than dePaola’s Legend of the Bluebonnet and Legend of the Indian Paintbrush.  dePaola’s simple drawings and the unique messages his books convey make them popular with teachers and parents, too.

De Paola wrote the Legend of the Bluebonnet twenty-eight years ago, but the story is as special now as when it was written.  She-Who-Is-Alone is a Comanche Indian living in Texas many years ago.  She is called She-Who-Is-Alone because everyone else in her family had died because of the drought.

In hope of breaking the drought, the tribe’s leaders said that the Great Spirit wanted tribe members to sacrifice their most prized possession.  She-Who-Is-Alone only had one possession, a doll her grandmother had made from buffalo skin.  The face was decorated with the juice of berries, and beautiful blue flowers were on her head.  The doll was all she has left of her family.

Creative Commons License photo credit: ruthieonart

In the night She-Who-Is-Alone slowly crept to the fire and threw her most prized possession into the flames.  When the ashes grew cold She-Who-Is-Alone threw them into the wind.  In the morning she could not believe what she was seeing. The hills were covered with beautiful blue flowers—the same color blue as the doll’s feathers.

Soon it started to rain and the drought was broken.  The tribe members changed She-Who-Is-Alone’s name to One-Who-Dearly-Loves-Her-People, and every spring the bluebonnets bloom to remind us of the sacrifice of one special young girl.

Little Gopher is the central character in the Legend of the Indian Paintbrush. Unlike the other boys, Little Gopher did not like to run, ride and play; his special talent was painting.  When he went to the hills to contemplate becoming a man, Little Gopher had a dream.  The vision told him to find a white buckskin and keep it.  One day he would paint a picture “that is as pure as the colors in the evening sky.”

Indian Paintbrush Washington Cascades
Creative Commons License photo credit: B Mully

Although he found the buckskin, Little Gopher could not find the right colors. However, one night a voice told him to go on top of a hill the next day at sunset. The voice said, “Because you have been faithful to the People and to your true gift, you shall find the colors you are seeking.” The next evening, Little Gopher found paintbrushes the colors of the sunset all over the hill, and he painted his masterpiece. When he returned to his tribe, Little Gopher left the paintbrushes behind.

The next morning the paintbrushes were all over the hills and had turned into beautiful flowers.  Little Gopher became known as “He-Who-Brought-the-Sunset-to-the-Earth.”  Being true to yourself and using the talents you have been given are wonderful messages for children.

Hopefully, all Texas children will become familiar with The Legend of the Bluebonnet and The Legend of the Indian Paintbrush and want to learn more about the stories unique to our state. With a state as big as Texas, there is so much to learn, and a great place to begin is at HMNS’ new exhibit, Texas! the exhibition, open now.