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…]


Related Posts:

Can Venus help us find exoplanets?…

This post is in response to loyal reader Jarman Day-Bohn’s question, which he left in a comment on the post “Today is transit day….”. Jarman asked:

How much do you think this [transit of Venus] will contribute to the current research experts are performing toward the study of possible earth-like planets out there? I know they were heavily using a technique measuring how much of a star’s light is blocked out by a planet to judge its size and other factors. Will this in any way help that process?

Great question Jarman, thanks for asking! Let’s look into that a little bit more.

But before we get too far into that, let’s think a little more about transits. Whether or not a transit occurs is all based on perspective. From Earth, only Mercury and Venus are interior planets- planets orbiting closer to the Sun- so they’re the only two that can we can see transit the Sun’s disk. If you were on Mars though, you could conceivably see Mercury, Venus, and Earth transits. And if you were on Pluto you could, in theory, see all eight planets transit the Sun. Remember though, as you get further away, even though more planets can be seen transitting the Sun from your perspective, you’re also getting further away, meaning the Sun is going to look smaller and smaller to you, as are the transitting planets. The Sun is only 93 million miles from the Earth (that’s really close astronomically speaking), so the enormous Sun, which is 1 million times larger in volume than the Earth, takes up a relatively large portion of the sky (~0.5 degrees). As you move further away from the Sun, its angular size in the sky will shrink. By the time you got to Pluto, which is 3.67 billion miles from the Sun, but still close astronomically speaking, our local star would look like a bright speck only ~0.01 degrees (50 times smaller than in the sky on Earth); probably something similar to the artist’s depiction below.

This artist’s depiction shows what the Sun might look like from an object, like Pluto, that’s in the solar system’s Kuiper Belt. Notice how the relatively close Sun differs from the background stars. Credit: NASA/JPL-Caltech/T. Pyle (SSC)

Now as you travel further from the Sun, let’s say to a planet orbiting another star, and look at our Sun, you could still in theory see all eight planets (and Pluto) transit the Sun, but now you’re trillions of miles from the Sun which is now just point of light in the sky, the same way other stars appear to us in the nighttime sky. The really astounding thing about looking at the other stars in our galaxy (Note: every star you see in the nighttime sky is in the Milky Way, we can’t resolve single stars in other galaxies.) is that they are so incredibly far away that no matter how large a telescope we use, we can never see the disk of the star like we can with the Sun, it just won’t have a large enough angular size. Now matter what, even through the Hubble Space Telescope, stars look like pinpricks of light that astronomers call “point sources“. Which makes actually seeing a planet transit a star, like we can see with Venus, impossible. I’ll remind you that in a previous post entitled “Baseballs, not umbrellas…” I explained the continuing search for exoplanets and covered all three of the main techniques which scientists employ to find these elusive celestial bodies around our galaxy. As Jarman indicated, the most successful and commonly used method of detection is the “transit method”. This is the method that NASA’s Kepler mission has already used to find the first Earth-like rocky exoplanet. However, since I just explained that we can’t actually see the exoplanet transitting the distant star, the only way we can detect the transit is by recording the change in light we see as the exoplanet crosses in front of the star. But again, we don’t actually “see” the transit happen, we just observe the dip in the brightness given off by the star. Similarly, if you were on a ship off the shore and someone walked in front of the lighthouse beacon, you wouldn’t be able to see the person, but you might be able to record the drop in brightness as they walked by.

This artist’s idea of NASA’s Kepler mission looking for exoplanets illustrates the main technique that scientists hope to use to find planets orbiting other stars- called the transit method- but no matter the size of the telescope we can’t actually see the exoplanets transitting the disk of the star. Credit:

But now let’s get back to Jarman’s actual question: can a transit in our solar system, like that of Venus, help scientists to find planets transitting other stars? It might. The transit of Venus is a well-documented and well-understood phenomenon, which scientists have been able to accurately predict and observe at least 6 times in the last 400 years. As I explained in “Looking to launch and preparing for transit…“, the transit of Venus helped us to determine the size of our solar system. And since we’ve actually been able to explore our solar system and have a very good grasp of the size of Venus and the Sun and the distances between the Earth and each of them, we can use the transit of Venus as a calibration tool in our search for extrasolar planets. For instance, scientist can say a planet like Venus, which is so big, transitting in front of a star like the Sun, which is so big, would cause a drop in brightness of this much at this distance. Then we can scale that distance out to other stars and we get some idea of what we need to look for in our search for exoplanets.

So Jarman, there’s your answer: while the spectacular crossing of Venus may not lead to groundbreaking new methods to find exoplanets, it does give us a rare opportunity to view a transit (that involves objects we know a lot about) and use that as a reference point as we continue our search!

Thanks again to Jarman for posing this question and if you, like Jarman, have a question that you’d like to have answered, please leave a comment or use the “Contact astroian” tab at the top of the page to send me an email!


Related Posts:

Around the horn…

Although this week’s title is mostly inspired by the fast approach of baseball season, it also characterizes this week’s post. This week’s post is going to update 4 updates in NASA news with a baseball flair. For those of you who aren’t sports fan, an “around the horn” is usually how the baseball gets back to the pitcher after a batter strikes out during an inning. The catcher, having caught the ball, throws it to the third baseman, who throws it to the second baseman, who throws it to the shortstop who throws it back to the pitcher. So here we go:

Catcher) This blog and many like it on the web have been chronicling the final days of the U.S. Space Shuttle program. As I previously posted, Discovery was supposed to have made its final launch back in November, but akin to likes of Michael Jordan and Brett Favre (a joke I prophetically made in the November post), Discovery was approved for “one last” launch to deliver the final module of the U.S. section of the International Space Station. Here are some very patriot photos of Discovery’s launch and a very cool video captured by a passenger on a plane flying over Cape Canaveral.

Third Baseman) Another key objective of Discovery’s final mission is to carry the first humanoid robot to space, Robonaut2. As previously reported here in “Thank you very much, Mr. Robonauto…”, R2 (a joint venture between NASA and GM) is a half-bodied (torso up) humanoid robot with 38 processors and extremely impressive dexterity. R2 is expected to aid the six-member Discovery crew in their mission as part of its preliminary flight testing.  Although some are ripping NASA for overplaying R2’s usefulness and potential, NASA and GM have maintained that R2 is a huge step in advancing space exploration. This wanna-be astronaut just hopes that there’s still room for humans in space.

Second Baseman) NASA has delayed launch of its Glory mission, a low-earth satellite satellite that will focus on collecting “data on the properties of aerosols, including black carbon, in the Earth’s atmosphere” and “data on solar irradiance for the long-term effects on the Earth climate record”. Although it’s cool that this mission exists and that NASA is taking an active approach on climate research, the real interesting side note on this is that the satellite is supposed to launched on an Orbital Sciences Taurus XL rocket. As previously posted a number of times, the ever-heating “commercial space race” is well underway and success of Orbital Sciences’ Taurus XL could make things interesting between them and 2010 Limitless Award Winner Elon Musk’s SpaceX Corporation. In any case, every successful launch of a commercial lift vehicle puts another nail in the coffin of the Space Shuttle program, despite NASA’s proposal to extend it to 2017.

Shortstop) The Kepler mission, NASA’s exoplanet-probing telescope previously highlighted in “The first of many…” and “Baseballs, not umbrellas…”, has really gotten going now. In the past month, the NASA spacecraft has discovered five Earth-sized planets in our Milky Way galaxy. All of these planets are orbiting stars smaller and cooler than our Sun, but they do lie in the “habitable zone”, meaning they are all at distances from their parent stars that make the presence of liquid water possible. Kepler also discovered six larger-than-Earth planets orbiting a single Sun-sized star roughly 2,000 lightyears away. More and more findings like this could prove that stellar planetary systems (that is, stars with multiple planets like our own) may not be unlikely in the galaxy. And, of course, every exoplanet we find makes the probability of finding life elsewhere in the universe seem more realistic. To date, Kepler has identified over 1,200 planet candidates and confirmed 15 (the current total confirmed count is 528 exoplanets orbiting 442 different stars).


Related Posts:

The first of many…

Last week, NASA’s Kepler spacecraft discovered the first terrestrial (rocky) exoplanet orbiting a star about 560 lightyears away from Earth. This marks the first rocky, Earth-like planet found outside of our solar system and brings the total exoplanet count to a whopping 500. If you’re really interested in keeping up to date with the exoplanet count, I suggest checking out the Jet Propulsion Laboratory’s Planetquest that features an up-to-date exoplanet count  (they even offer a downloadable exoplanet counter widget) and fun interactive exoplanet activities and multimedia.

The new exoplanet, dubbed Kepler-10b, is the tenth exoplanet discovered by the Kepler mission, a mission which is proving capable of fulfilling its primary science objective to “Determine the abundance of terrestrial and larger planets in or near the habitable zone of a wide variety of stars” [1]. Unfortunately for Kepler though, as I reviewed in a previous post, the techniques we currently use to find exoplanets make it much easier to find larger planets (usually gas giants like Jupiter) than smaller rocky ones. Unfortunately, Kepler-10b is not in its star’s “habitable zone” or the correct distance from the star for water ice to exist on the planet; it’s twenty times closer to its star than Mercury is to the Sun. If a planet is too close to its star then all the water will evaporate and if its too far away it will all freeze. Scientists are assuming that planets with liquid water will have the highest chance of supporting life (like Earth which has a surface 70% covered by water). Kepler-10b’s extreme proximity to its parent star probably means that the surface of the planet is either scorched arid rock or possibly even covered with a layer of molten lava. In any case though, the discovery of this first terrestrial world is a great sign of things to come.

Shifting gears…

January has been a big month for crazy astronomical stories gone awry in the media. After the zodiac controversy that hit last week, this week a new craze has exploded after an interview with Dr. Brad Carter, Senior Lecturer of Physics at the University of Southern Queensland in Australia, published by In the interview, Dr. Carter talks about the expected supernova of the red giant star Betelgeuse (yes, it’s pronounced Beetlejuice…Beetlejuice, Beetlejuice). The article is riddled with inaccuracies and just downright wrong information. First of all, the writer, Claire Connelly, tries to inaccurately spin the story to appeal to Star Wars fans by making it seem like Earth will have two suns like the fictional world of Tatooine. Connelly says, “[I]t’s not just a figment of George Lucas’s imagination – twin suns are real. And here’s the big news – they could be coming to Earth. Yes, any day now we see a second sun light up the sky, if only for a matter of weeks.” While it is true that “twin suns” are in fact real (we know that roughly half of the stars in are galaxy exist in multiple-star systems), there is no way that the Earth can ever have twin Suns. Binary star systems usually form together, meaning the other Sun would have to have been created at the same time as our Sun (which obviously did not happen). I suppose a very rare case could occur where a star comes in close contact to another star and the effect of gravity causes them to fall into mutual orbit, but the Sun is way too far away from any other stars (the closest is 4.2 lightyears away) and if that ever did happen, the planets in our solar system would probably be flung out of their orbits altogether. While it’s true that when Betelegeuse does supernova (which could be tomorrow or in another million years), we will be able to see the supernova during the daylight hours (much like how we see Venus in the early morning hours before sunrise), it doesn’t mean that we’ll have two suns or that “one day, night will become day for several weeks on Earth.” The supernova will just look like an extremely bright star in the sky that will be visible at night and during the day for a few weeks. The article goes on to make allusions to the Mayan 2012 apocalypse predictions, imply associations between the word “Betelgeuse” and the devil, and to erroneously state that Betelgeuse is the “second biggest star in the universe” (it’s the second largest in its constellation, Orion).

Of course, even though several reputable new sources quickly tried to convince people that the claims in this article were nonsense (see FoxNews and Discovery), others quickly tried to jump on the lead and continued to erroneously echo the story (I’m looking at you, Huffington Post). Just another example of how poor journalism can fuel public paranoia and misinformation.

Betelgeuse is the red giant star that makes up Orion’s left shoulder in the sky.

Anyways, to wrap up this post, I figured I’d give you some fun information about Betelgeuse and its constellation Orion. Betelgeuse is the reddish star seen in the upper left of Orion, commonly seen as his left shoulder (see image above). It’s roughly 10 million years old and large enough that if it replaced our Sun, it would extend all the way out past the orbit of Jupiter. Orion, known as the famous hunter of the Greeks who was killed by Scorpio because he refused to acknowledge the gods, is also known by several other names around the world. In Egyptian lore, he is the god Osiris, who rules over the afterlife and judges the dead. In Arabic mythology, he is known as Al-Jabbar or The Giant and the name Betelgeuse, which comes to us from Arabic like many other star names, is said to loosely translate to “the Giant’s armpit”.


Related Posts:

%d bloggers like this: