It seems the sky is falling…

Imagine driving in your car on a lovely Friday morning and seeing a flaming ball of death streaking across the sky and coming, as best you can tell, right at you.

This view from a Russian dashboard camera shows a terrifying view of the fireball as the meteoroid entered the atmosphere and hurtled over the city of Chelyabinsk. Credit: Discovery News

That’s what terrified citizens in the lovely Russian city of Chelyabinsk experienced on the morning of Friday, February 15. The multitude of videos and photos of this meteor are simply horrifying since many of them give the impression that this huge chunk of flaming interplanetary death is about to smash right into the camera. Not only did this fireball make a scary visual impression, but it packed a very literal punch as well. As the meteoroid hurtled through the atmosphere at 40,000 mph, the heat and pressure it felt caused it to break apart with a huge amount of energy, the equivalent of 470 kilotons of TNT (or 30-40 times the power of the atomic bomb dropped on Hiroshima). The deposition of that huge amount of energy into the sky caused a pressure wave that blasted the city. Over 1,000 people were injured by the blast, mostly due to cuts and scrapes from glass as windows shattered. Scientists have now come to the conclusion that the initial object was only 17 meters wide– that’s about the size of a tractor trailer. That’s pretty small cosmically speaking. Imagine the damage that could have been inflicted if something larger had hit the atmosphere. The last time a meteor had significant large-scale impact was in 1908, again in Russia. This impact, known as the “Tunguska event“, is the largest impact ever recorded- 20-30 times larger than the one that happened this month. This meteoroid, which is estimated to have been about 100 meters wide (the size of a football field), blew up in the air and released 10-15 megatons of energy, leveling 830 square miles of trees. Witnesses to the event said that the heat and pressure from the explosion made their skin feel like it was on fire.

The 1908 Tunguska event, the largest impact near or on Earth ever recorded, leveled trees over 830 square miles. Credit: nightsky.org

Luckily the 2013 Russian meteor was much smaller, so windows got knocked out but buildings weren’t leveled. The object was actually so small that astronomers didn’t even see it coming. NASA has a whole division of people who track objects that could potentially come close to Earth, it’s known as the Near-Earth Object Program. Unfortunately, for scientists to be able to see an object it needs to be large enough to reflect an observable amount of light. That didn’t happen here.

The meteor also came as a bit of a shock since scientists were so focused on another Near-Earth Object called 2012 DA14. This 45-meter wide asteroid was scheduled for a flyby of Earth on the same day, February 15. This relatively small piece of space rock flew closer to the Earth than any other celestial body. It was 17,200 miles away at its closest approach, that’s closer than satellites in geosynchronous orbit and much, much closer than the Moon. Although scientists were certain that DA14 wouldn’t impact the Earth, they were very excited to use the close flyby as an opportunity to study the asteroid.

This collage of 72 individual radar-generated images of asteroid 2012 DA14 was created using data from NASA’s 230-foot Deep Space Network antenna at Goldstone, CA. Credit: NASA

Of course it was ironic that after weeks of assuring the public that there was no threat of an impact from DA14 another huge impact happened in Russia the same day. Scientists from NASA’s Meteoroid Environment Office concluded that the Russian meteor and DA14 were totally unrelated, having come from two very distinct trajectories/orbits. This means it was a huge cosmic coincidence that they just happened to occur on the same day…weird.

This plot of the orbits of the Russian meteor and asteroid 2012 DA14 show that the two bodies came from very different parts of the solar system and were unrelated. The Russian meteoroid most likely originated from the Asteroid Belt out past Mars. Credit: NASA/Space.com

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Each in a class of their own…

Ever wonder what makes stars different from one another? Lots of factors can come into play: size, composition, temperature, and age to name a few. Thankfully many stars are similar and can be grouped together by similarities. So here let’s talk about the history and science of stellar classification.

Shedding some light on spectra

In the middle of the 19th century, a German physicist by the name of Gustav Kirchhoff was doing a lot of research into the field of spectroscopy; collaborating closely with Robert Bunsen, inventor of the best piece of scientific equipment high schoolers are allowed to use. Kirchhoff in his research, came to the conclusion that spectroscopy was governed by three basic laws. These are known today as “Kirchhoff’s laws of spectroscopy” (not to be confused his circuit lawslaw of thermochemistry, or law of thermal radiation– basically this guy made more laws than Congress). Kirchhoff’s laws of spectroscopy dictate that:

  1. A solid (or liquid or gas under high pressure) will give off a continuous spectrum.
  2. A gas under low pressure (i.e. most gases we know of) will produce bright, discrete lines known as an emission spectrum.
  3. If you look at a source of a continuous spectrum from behind a source of an emission spectrum, you will see what looks like a continuous spectrum with black lines missing from it; think of if you took the emission spectrum and subtracted it from the continuous spectrum. This is called an absorption spectrum.

An example of Kirchhoff’s laws of spectroscopy. On the left you see an example of a continuous spectrum (Law 1) and an emission spectrum (Law 2) on the right. In the middle is an example of an absorption spectrum (Law 3), basically the removal of the emission line from the continuous spectrum. Credit: Penn State

Kirchhoff asserted that the wavelength or location of these emission or absorption lines was determined by what atoms or molecules were present in the source. This is true because each element or molecule has a unique atomic spectrum or signature. At the time that Kirchhoff came up with these laws scientists had yet to crack the secret of the internal structure of the atom. Meaning Kirchhoff made these laws based on purely on experimentation. It took another half a century for Niels Bohr to come up with a correct model of the atom that concluded the existence of discrete energy levels that successfully explained Kirchhoff’s emission and absorption lines (and later led to the formulation of quantum mechanics).

Enter the Harem

So back towards the end of the 19th century, a man by the name of Edward Pickering was the director of the Harvard College Observatory. Mr. Pickering decided to take it amongst himself to obtain spectra of as many stars as he could and then index and classify them. So Pickering did what any good scientist would do, he began to collect data. But as you well know, there are a lot of stars in the sky, so before he knew it he was inundated with tons of photographic plates (if you thought film was bad, its predecessor was worse- these plates were usually large heavy pieces of glass mixed with silver salts) containing stellar spectra. Legend has it that Pickering was getting so aggravated by the incompetence of his male research assistants that he exclaimed that his maid could do a better job. So he hired her. Her name was Williamina Fleming and along with Pickering she helped to publish the Draper Catalogue of Stellar Spectra (named in honor of Henry Draper, the first man to take the spectrum of a star on a photographic plate), which had classifications for 10,351 different stars. Once Fleming left Pickering’s service, he hired several other women assistants. Out of this group of women, which became known officially as the “Harvard Computers”, but commonly as “Pickering’s Harem”, came some of the greatest early female astronomers, including Annie Jump CannonHenrietta Swan Leavitt, and Antonia Maury. The initial version of this catalog, published from 1918 to 1924 in 9 volumes, included the positions, magnitudes, and spectral classifications of over 225,000 stars.

Edward Pickering and his “harem” outside a Harvard building in 1913. Annie Jump Cannon stands two to the right of Pickering. Credit: UC- Berkeley

Differentiating the spectral classes

Alright, so how does that help astronomers? Well, in essence a star is a gas under high pressure, meaning it should give off a continuous spectrum according to Kirchhoff’s first law. But the outer layers of a star’s “atmosphere”, called the corona, is a gas under low pressure- meaning we actually see an absorption spectrum (Law 3). (In fact, it was the unexpected discovery of this absorption spectrum that helped us to realize that our Sun had an “atmosphere” or outer layer of hot gas surrounding it.) Since stars of the same size and mass are made up of pretty much the same stuff, they have similar spectra. In fact, this is how astronomers classify stars, by their spectral class. The different stellar spectral classes are O, B, A, F, G, K, and M. Type O stars are the hottest and Type M stars are the coolest. Each spectral class or spectral type has a unique spectrum.

Recreated stellar spectra of each spectral type (from top to bottom): O, B, A, F, G, K, M. Credit: ESA

With a name like that…

Now, like a lot of things in astronomy, this naming scheme is totally absurd and illogical. I wish I had a better explanation for why we have this naming scheme, but basically it’s a historical holdout from back when astronomers started classifying stars without really understanding them. Remember the Harem? Well in the first publication of the catalog in 1890, Williamina Fleming did most of the classification. She used a classification system that had been developed a few decades earlier by the Italian astronomer Angelo Secchi. Since she had so many stars, she took Secchi’s five classes and stretched them out to encompass fourteen classes from A to N. Then she added three more categories (O, P, Q) to encompass stars that would not have fit Secchi’s scheme. A through Q made sense. But then in 1897, Antonia Maury was working on a different set of stars and decided to reclassify what Fleming had done. So she scrapped the letters and made 22 classes from I to XXII…still made sense. Unfortunately, in her rearranging of Fleming’s classes, she wasn’t paying attention to the letters and moved some around, hence O and B moving towards the front.

Finally in 1901, Annie Jump Cannon (probably the most famous and accomplished of the Harem) was cataloging and decided to go back to the letter system and dropped all the letters except O, B, A, F, G, K, and M in that order. Why? I have no idea. For some reason after Ms. Jump Cannon came up with her system they had had enough reclassification and no one suggested, “Hey maybe we should have these make some kind of logical sense.” Astronomers can be infuriating sometimes.  The final crazy product is known today as the Harvard Spectral Classification. So, if you need a way to try to remember Ms. Jump Cannon’s crazy archaic classes, try “OBA Fine Gal (or Guy), Kiss Me!” Of course, the cockamamie lettering system wasn’t enough, Ms. Jump Cannon then needed to add ten subclasses from 0 to 9 for each letter. Meaning not only is a B-type star hotter than a K-type star, but a B1 star is hotter than a B5. Our star, the Sun, is a G2, meaning it’s pretty much right in the middle of the stellar pack.

Digging even deeper

But somehow the crazy letter and number combination still wasn’t quite exact enough. In 1943, three astronomers from the Yerkes Observatory in Wisconsin came up with another classification system that focused not only on the surface temperature of a star (which the Harvard Classification does), but also on the luminosity (or brightness). Basically, you can have a really big red giant star and a teeny tiny white dwarf star that are the same temperature and therefore have similar emission lines. However, you can look at how sharp those emission lines are and determine the surface gravity or pressure that that star must have. When introducing this new factor into the equation, the Yerkes astronomers came up with seven (I-VII) new classes that basically help to dictate what stage of life a star is in.

To try to help this make some visual sense, astronomers have developed a graph called the Hertzsprung-Russell diagram that correlates how bright a star is, how hot it is, and what spectral class it’s in. This pretty ingenious and very common graph helps to simplify a vast amount of knowledge. It’s really pretty obvious how the groups appear when looking at a filled out H-R diagram. Most stars, like our Sun (which is a G2V), are in class V, meaning they are still on the “Main Sequence” and are still fusing hydrogen into helium. As stars live and evolve, they move off of the main sequence and into other branches of the H-R diagram. Can you pick out where the Sun would be on this H-R diagram below?

A Hertzsprung-Russell diagram showing the major classes of stars. The temperature (and spectral classes) run from hottest to coldest, left to right. Generally size decreases from top to bottom. The “Main Sequence” is the diagonal line running through the middle, with the other evolutionary branches around it. Credit: Wikipedia

The Cosmic Distance Ladder, part 1…

For the past few months, I’ve been spending a lot of time in my position as Manager of the UNH Observatory, in helping to prepare for the 2012 New England Fall Astronomy Festival. The event, lovingly known as NEFAF, is a family-friendly astronomy-related event that will be hosted by the UNH Physics Department in partnership with the New Hampshire Astronomical Society. As you can imagine, this is quite an undertaking, but in an incredibly exciting turn of events, we just found out that Dr. Alex Filippenko, noted astronomer from UC-Berkeley and member of the research team that won the 2011 Nobel Prize in Physics, will be giving the keynote talk at NEFAF 2012! In addition to being a highly acclaimed professor, Filippenko is also the co-author of an extremely popular astronomy textbook and a frequent contributor to the documentary series The Universe on The History Channel.

Dr. Alex Filippenko, the newly announced keynote speaker for the 2012 New England Fall Astronomy Festival to be held at the UNH Observatory.

That extremely exciting news has inspired me to do a couple of posts about the expansion of the universe, the area of research that Dr. Filippenko works on. But before we can really get into talking about that, we need to cover a very basic aspect of astronomy, but something that most non-astronomers don’t really know about. I was at a public session at the Observatory this weekend when a guest who had never studied astronomy before asked me what she thought might be an “ignorant” question: she wanted to know how exactly astronomers knew the distances to objects in space. This is by no means an ignorant question, in fact it’s a very fundamental and very involved question that really gets at the very nature of astronomy.

Astronomy by definition is an observational science. Unlike many other scientific disciplines, astronomers can’t really do experiments in a laboratory (although some do). But the stereotypical astronomer can’t throw his subject (a star or galaxy) on a lab bench and dissect it or set up an experiment to test it, so astronomers need to observe and record data. Okay, so we observe light, that tells us what something looks like, where it is, and how bright it is. Big deal, is that really that helpful scientifically. Well, not really. So we have to come up with ways to get more information from observing the light. The main way we do that is by breaking the light up in a spectrometer, an instrument that breaks light down into a spectrum of colors. This breakdown of light can reveal an abundance of new information including what the object is made up of, how hot it is, how fast and in what direction it’s moving, how old it is, and more.

The question of how astronomers calculate the distance to an astronomical object varies depending on how far away the object is. Because most of these techniques only work up to a certain distance, there is actually a progression of different approaches that astronomers use to measure distance to celestial objects. This list of methods of measuring astronomical distances is known as the Cosmic Distance Ladder (or less poetically, the extragalactic distance scale).

A graphical representation of the distance-measuring techniques that make up the Cosmic Distance Ladder. “1 A.U.” is 1 astronomical unit or approximately 93 million miles, the distance from the Earth to the Sun. A “pc” is a parsec, equal to 3.2 lightyears (206,265 A.U.) or about 20 trillion miles. “Mpc” stands for “Megaparsec” or millions of parsecs. Credit: University of Rochester

In our solar system

The first step in our exploration of the universe was to our own celestial neighborhood. The first step was precise measurement of the size scale of the solar system, which started with the determination of the distance between the Earth and the Sun. As I’ve explained before, this measurement was originally calculated via observations of transits of the planet Venus across the disk of the Sun. Early on in the 20th century, observations of asteroids also played an important role in this measurement. But today the distance from the Earth to the Sun, defined as 1 Astronomical Unit or “AU”, is measured with high precision using radar ranging. By bouncing a radar beam off another planet, usually Venus, and measuring the time that beam takes to return to Earth, scientists can very accurately determine the difference in the size of the two planets’ orbits. Using that difference and the ratio of the two orbital sizes, we can very easily calculate the distance the Earth must be from the Sun. We use a similar process even closer to home. During the Apollo missions of the late 1960s-early 1970s, astronauts deployed the lunar laser ranging experiments, arrays of mirrors that allowed scientists to measure the distance to Earth’s only natural satellite with extreme precision using lasers. This radar ranging is how we’ve calculated the distance to most of the objects in our solar system. More recently, we can also use spacecraft in orbit around other planets as a tape measure by measuring the time it takes for a signal to travel from the spacecraft to its controllers on Earth.

Another  way we can get measure the distance to the planets and to nearby stars is a phenomenon known as stellar parallax. This method is less accurate than radar ranging for planets, but is very good for stars in our local stellar neighborhood. Parallax is something you experience almost every day. Hold your thumb up at arm’s length. Close one eye, then open that one and close the other. Notice how your thumb appears to shift with respect to the objects far off in the distance? That’s parallax! Astronomers take measurements of a planet or star a two points in Earth’s orbit (6 months apart) and measure the angular shift of an object with respect to the background stars between those two measurements. Then, because we know the distance the Earth is from the Sun, we can use some basic geometry to calculate the distance to that star or planet in the foreground.

This diagram shows how parallax is used to find the distance to planets in our solar system and nearby stars. Scientists make two observations 6 months apart, measuring the angle that an object (the red dot) makes with regard to the background stars between the two observations. Then using the distance from the Earth to the Sun (1 A.U.) and some simple geometry, the distance to the object (d) can be calculated. Credit: Hyperphysics

It was using parallax, that Italian astronomer Giovanni Domenico Cassini was able to roughly calculate the distance to Mars in 1672. His calculation was a little bit off though, because instead of taking two measurements 6 months apart, he sent his colleague to Cayenne, French Guiana (on the northern coast of South America) to make observations while he stayed in Paris. Then Cassini could make the same parallax calculation using the known distance between the two observation points (~4400 miles) instead of the distance from the Sun to the Earth. This single direct measurement of the distance to Mars, which is now easily calculated and heavily used by missions such as Curiosity, actually allowed for the calculation of the distances to all the planets. Since geometry and Kepler’s Laws governed the basic ratios the Sun-planet distances, you only needed to measure one Earth-planet distance to be able to easily calculate them all. This major contribution and several others in planetary science (including the discovery of four of Saturn’s moons and joint discovery of Jupiter’s Great Red Spot) prompted NASA to name the Saturn-bound spacecraft mission after the him.

Giovanni Domenico Cassini

[To be continued…]

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