Part 3 of the Cosmic Distance Ladder series is a little longer than the previous two. We will see which tools astronomers use to find the distances to objects much beyond our own home galaxy. There will be some discussion of supernovae and black holes.
The first part of this post advances up the distance ladder by telling the remarkable story of Henrietta Swan Leavitt. Her contributions to the study of Cepheid variable stars led to a method to know intergalactic distances millions of light years from our Milky Way. The previous Cosmic Distance Ladder – Part 2 discussed stellar parallax and main sequence fitting as methods to determine distances to objects within the confines of the Milky Way vicinity. If you wish to see parts 1 & 2, they are linked here and here.
The Harvard Calculators
Henrietta was the daughter of Congregational church minister George Roswell Leavitt and Henrietta Swan. She was born in Lancaster, Massachusetts. She attended Oberlin College, and graduated from Radcliffe College, with a bachelor’s degree in 1892. It wasn’t until her fourth year of college that Leavitt took a course in astronomy. She earned an A–.
Charles Pickering was the director of the Harvard Observatory from 1877 to 1919. His work included obtaining many large window pane sized photographic plates through the Harvard telescope of a variety of objects. As with astronomical research today, much time and expense was required to view and analyze the data recorded on the glass plates. Someone needed to study the photographs. That is where Henrietta and several other women come into this story.
Leavitt began work in 1893 at Harvard College Observatory as one of the women human ‘computers’ brought in by Edward Charles Pickering to measure and catalog the brightness of stars in the observatory’s photographic plate collection. In the early 1900s, women were not allowed to operate telescopes. Among these women were Williamina Fleming, Annie Jump Cannon, Henrietta Swan Leavitt and Antonia Maury. This staff came to be known as “Pickering’s Harem” or, more respectfully, as the Harvard Computers. This was an example of what has been identified as the “harem effect” in the history and sociology of science.
It seems that several factors contributed to Pickering’s decision to hire women instead of men. Among them was the fact that men were paid much more than women, so he could employ more staff with the same budget. This was relevant in a time when the amount of astronomical data was surpassing the capacity of the observatories to process it.
The first woman hired was Williamina Fleming, who was working as a maid for Pickering. It seems that Pickering was increasingly frustrated with his male assistants and declared that even his maid could do a better job. Apparently he was not mistaken, as Fleming undertook her assigned chores efficiently.
Henrietta was hired to analyze photographic plates at the rate of $0.30 per hour. Her work with 1777 variables in the Magellanic Clouds was published in 1908 and is in the Annals of the Astronomical Observatory of Harvard College.
Biographical information on many of these women can be found in this link.
The video clip below is about a play written about Henrietta Leavitt by Lauren Gunderson’s called “Silent Sky”.
Here is another video I found interesting about Henrietta Leavitt.
What Were Leavitt’s Findings?
Below is an example of the types of plates viewed and analyzed by Leavitt. This one has Cepheids marked between the — — lines. The stars are barely visible. It was painstaking and slow work.
From plates of the Magellanic Clouds, neighbors to our Milky Way, she identified the class of Cepheid variables. She noted that the brighter ones varied in brightness in a periodic way that took longer than the less bright ones. Since they were all essentially at the same distance from Earth, the brightness was only a function of the luminosity of the Cepheid, not the distance to it. She plotted this relationship as below. The max and min lines are when the variable was observed to be at maximum or minimum brightness.
By knowing the number of days it takes to pass through a cycle of brightness, the intrinsic luminosity can be inferred. Knowing the apparent luminosity for a Cepheid in a more distant galaxy, and the luminosity for one in a closer known distance as in the Magellanic Clouds, the distance can be calculated to the farther one. This is basically the headlight method described in the previous diary.
In the 1920’s Edwin Hubble detected Cepheids in the Andromeda nebula, M31 and the Triangulum nebula M33. Using these he determined that their distances were 900,000 and 850,000 light years respectively. He thus established conclusively that these “spiral nebulae” were in fact other galaxies and not part of our Milky Way. This was a momentous discovery and dramatically expanded the scale of he known Universe.
Today, the Hubble Space Telescope is able to record images of Cepheids with electronic CCD cameras. The image below is an example of one such image. There is an arrow indicating the Cepheid. Note the dates of each image.
Difficulties and Large Errors
There are inherent difficulties which were part of this method. Astronomers did not know the actual distance to the closer Cepheids with much accuracy. And the viewing of the photographic plates led to uncertainties judging luminosities. Hubble’s work was off by a factor of at least 2x.
Cepheid variable stars were the key instrument in Edwin Hubble’s 1923 conclusion that M31 (Andromeda) was an external galaxy, as opposed to a smaller nebula within the Milky Way. He was able to calculate the distance of M31 to 285 Kpc, today’s value being 770 Kpc. As detected thus far, NGC 3370, a spiral galaxy in the constellation Leo, contains the farthest Cepheids yet found at a distance of 29 Mpc. Cepheid variable stars are in no way perfect distance markers: at nearby galaxies they have an error of about 7% and up to a 15% error for the most distant.
Supernovae – Very Bright Sources
Another source of light which varies, though not in a cyclical way, is the supernova. It is inherently very much brighter than Cepheids. Supernovas are one of the most violent events in the universe, and the force of the explosion generates a blinding flash of radiation, as well as shock waves analogous to sonic booms. They are the evidence of the cataclysmic explosion of a star at the end of a stage of its life. Here is a step-by-step tutorial leading up to the event. In that process, the brightness increases by many orders of magnitude over the course of a few days. It then begins a slower decrease in brightness. Below is the light curve of supernova 1991T with values measured by the two astronomers who first reported it. This is a typical light curve showing how he light output varies over the course of days.
Below that is an animation of a supernova explosion modeled by the Chandra X-Ray Observatory.
Type II and Type I Supernovae
Type II supernovae occur in regions with lots of bright, young stars, such as the spiral arms of galaxies. They apparently do not occur in elliptical galaxies, which are dominated by old, low-mass stars. Since bright young stars are typically stars with masses greater than about 10 times the mass of the sun, this and other evidence led to the conclusion that Type II supernovae are produced by massive stars. Some Type I supernovas show many of the characteristics of Type II supernovas. These supernovas, called Type Ib and Type Ic, apparently differ from Type II because they lost their outer hydrogen envelope prior to the explosion. The hydrogen envelope could have been lost by a vigorous outflow of matter prior to the explosion, or because it was pulled away by a companion star.
White dwarf stars are composed of mostly carbon and oxygen. They are considered a very stable star, as long as their mass remains below 1.4 solar masses, the Chandrasekhar limit.
If, however, accretion of matter from a companion star or the merger with another white dwarf, push a white dwarf star over the Chandrasekhar limit of 1.4 solar masses, the temperature in the core of the white dwarf will rise, triggering explosive nuclear fusion reactions that release an enormous amount of energy. The star explodes in about ten seconds, leaving no remnant. The expanding cloud of ejecta glows brightly for many weeks as radioactive nickel produced in the explosion decays into cobalt and then iron.
Here is another animation from the Chandra X-Ray Observatory showing how two orbiting white dwarfs can lead to a supernova. This is a very informative animation worth watching.
The most notable supernova of recent times is sn1987a in the Large Magellanic Cloud. Its outburst sent a flux of neutrinos through two detectors just before the supernova was visually observed. It will be 25 yrs since it erupted this coming February. It remains one of the most studied supernova. This image shows the effects of the powerful shockwaves moving through the interstellar gas and dust. Hot spots of million degree gas shine in a necklace ring in this Hubble image.
Since Type Ia supernova form from the same type of star at about 1.4 solar masses, they are all of similar luminosity. The conveniently allows them to be used as the Standard Candle for distance determinations. And because they are so incredibly bright at their peak, they can be seen at huge distances in their host galaxies. It is the same type of argument used for the headlights of cars comparison. You judge how far away they are by knowing how bright they are at a distance. It is a comparison of absolute and apparent magnitudes.
Supernova Remnants and Black Holes
One of the historically significant supernovae is attributed to Tycho Brahe in 1572. In his own words…
“On the 11th day of November in the evening after sunset, I was contemplating the stars in a clear sky. I noticed that a new and unusual star, surpassing the other stars in brilliancy, was shining almost directly above my head; and since I had, from boyhood, known all the stars of the heavens perfectly, it was quite evident to me that there had never been any star in that place of the sky, even the smallest, to say nothing of a star so conspicuous and bright as this. I wqs so astonished of this sight that I was not ashamed to doubt the trustworthyness of my own eyes. But when I observed that others, on having the place pointed out to them, could see that there was really a star there, I had no further doubts. A miracle indeed, one that has never been prevoiously seen before our time, in any age since the beginning of the world.”
Today, Chandra provides us with an incredible view in x-ray of Tycho’s supernova. The image is zoomable.
And there is the Crab Nebula supernova explosion of 1054. It might have been as bright as the full Moon. It was probably also recorded by Anasazi Indian artists in present-day Arizona and New Mexico from findings in Navaho Canyon and White Mesa as well as in the Chaco Canyon National Park. Thanks again to Chandra Observatory for this April 2011 animated series of images taken over a 7 month period showing subtle movements at the core. The remnant core pulsar of this supernova at the center of the Crab Nebula is a neutron star that spins around about 30 times a second.
Finally, supernovae can also create a black hole at their center. The black hole is not part of the distance ladder. You can’t see it. It is an incredible object worthy of a blog post of its own. This final animation from Chandra illustrates the event well.
What’s Next On the Ladder?
Next time, the subject will be the Expanding Universe as described by Hubble’s Law. I hope to see you then.