How Do We Measure Distance in the Universe?

How Do We Measure Distance in the Universe?

This star is X light-years away, that galaxy is X million light-years away. That beginning the Universe is X billion light-years away. But how do astronomers know?

I’m perpetually in a state where I’m talking about objects which are unimaginably far away. It’s pretty much impossible to imagine how huge some our Universe is. Our brains can comprehend the distances around us, sort of, especially when we’ve got a pile of tools to help. We can measure our height with a tape measure, or the distance along the ground using an odometer. We can get a feel for how far away 100 kilometers is because we can drive it in a pretty short period of time.

But space is really big, and for most of us, our brains can’t comprehend the full awesomeness of the cosmos, let alone measure it. So how do astronomers figure out how far away everything is? How do they know how far away planets, stars, galaxies, and even the edge of the observable Universe is? Assuming it’s all trickery? You’re bang on.

Astronomers have a bag of remarkably clever tricks and techniques to measure distance in the Universe. For them, different distances require a different methodologies. Up close, they use trigonometry, using differences in angles to puzzle out distances. They also use a variety of standard candles, those are bright objects that generate a consistent amount of light, so you can tell how far away they are. At the furthest distances, astronomers use expansion of space itself to detect distances.

Fortunately, each of these methods overlap. So you can use trigonometry to test out the closest standard candles. And you can use the most distant standard candles to verify the biggest tools. Around our Solar System, and in our neighborhood of the galaxy, astronomers use trigonometry to discover the distance to objects.

They measure the location of a star in the sky at one point of the year, and then measure again 6 months later when the Earth is on the opposite side of the Solar System. The star will have moved a tiny amount in the sky, known as parallax. Because we know the distance from one side of the Earth’s orbit to the other, we can calculate the angles, and compute the distance to the star.

I’m sure you can spot the flaw, this method falls apart when the distance is so great that the star doesn’t appear to move at all. Fortunately, astronomers shift to a different method, observing a standard candle known as a Cepheid variable. These Cepheids are special stars that dim and brighten in a known pattern. If you can measure how quickly a Cepheid pulses, you can calculate its true luminosity, and therefore its distance.

Hubble Frontier Fields observing programme, which is using the magnifying power of enormous galaxy clusters to peer deep into the distant Universe.. Credit: NASA.
Hubble Frontier Fields observing programme, which is using the magnifying power of enormous galaxy clusters to peer deep into the distant Universe. Credit: NASA.

Cepheids let you measure distances to nearby galaxies. Out beyond a few dozen megaparsecs, you need another tool: supernovae. In a very special type of binary star system, one star dies and becomes a white dwarf, while the other star lives on. The white dwarf begins to feed material off the partner star until it hits exactly 1.4 times the mass of the Sun. At this point, it detonates as a Type 1A supernova, generating an explosion that can be seen halfway across the Universe. Because these stars always explode with exactly the same amount of material, we can detect how far away they are, and therefore their absolute brightness.

At the greatest scales, astronomers use the Hubble Constant. This is the discovery by Edwin Hubble that the Universe is expanding in all directions. The further you look, the faster galaxies are speeding away from us. By measuring the redshift of light from a galaxy, you can tell how fast it’s moving away from us, and thus its approximate distance. At the very end of this scale is the Cosmic Microwave Background Radiation, the edge of the observable Universe, and the limit of how far we can see.

Astronomers are always looking for new types of standard candles, and have discovered all kinds of clever ways to measure distance. They measure the clustering of galaxies, beams of microwave radiation from stars, and the surface of red giant stars – all in the hopes of verifying the cosmic distance ladder. Measuring distance has been one of the toughest problems for astronomers to crack and their solutions have been absolutely ingenious. Thanks to them, we can have a sense of scale for the cosmos around us.

What concept in astronomy do you have the hardest time holding in your brain? Tell us, in the comments below.

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What’s Going On Inside This New Kind of Variable Star?

Thirty-six of the stars in this open star cluster, NGC 3766, are a variable star never seen before. Observations were made with the European Southern Observatory's La Silla Observatory. Credit: ESO

A new kind of variable star — 36 of that type, in fact — has been found in a single star cluster. Astronomers don’t even have a name for the star type yet, but feel free to leave some suggestions in the comments!

For now, however, astronomers are wondering what the implications are for our understanding of the stellar interiors.

“The very existence of this new class of variable stars is a challenge to astrophysicists,” stated Sophie Saesen, an astronomer at Geneva Observatory who participated in the research.

“Current theoretical models predict that their light is not supposed to vary periodically at all, so our current efforts are focused on finding out more about the behaviour of this strange new type of star.”

The head-scratching began when astronomers used a European Southern Observatory telescope to gaze at the “Pearl Cluster” (NGC 3766), an open star cluster about 5,800 light years from Earth.

Over seven years of observations with the Leonhard Euler Telescope (taking periodic measurements of brightness), astronomers spotted 36 stars with variable periods of between 2 and 20 hours.

The four-foot (1.2-meter) Leonhard Euler Telescope at the European Southern Observatory. Credit: M. Tewes/ESO
The four-foot (1.2-meter) Leonhard Euler Telescope at the European Southern Observatory. Credit: M. Tewes/ESO

Variable stars have been known for centuries, and many of them are tracked by amateur organizations such as the American Association of Variable Observers. As best as astronomers can figure, the stars become brighter and dimmer due to changes on the inside — stellar vibrations or “quakes” studied under a field called asteroseismology.

A special type of variable stars, called Cepheid variables, can provide accurate measurements of distance since they have an established ratio between luminosity and the period of their variability.

Studying various types of variable stars has provided some insights.

“Asteroseismology of ß Cep[hei] stars, for example, has opened the doors in the past decade to study their interior rotation and convective core,” the astronomers stated in a paper on the research.

The variations in brightness can be interpreted as vibrations, or oscillations within the stars, using a technique called asteroseismology. The oscillations reveal information about the internal structure of the stars, in much the same way that seismologists use earthquakes to probe the Earth's interior. Credit: Kepler Astroseismology team.
The variations in brightness can be interpreted as vibrations, or oscillations within the stars, using a technique called asteroseismology. The oscillations reveal information about the internal structure of the stars, in much the same way that seismologists use earthquakes to probe the Earth’s interior. Credit: Kepler Astroseismology team.

Despite the well-known nature of variable stars, few of them have been studied in open clusters such as NGC 3766.

The reason is it takes a lot of telescope time to take a look at the star — sometimes, years. And time with telescopes is both expensive and precious, making it difficult to allocate the time required.

“Stellar clusters are ideal environments to study stellar variability because some basic properties and the evolutionary status of individual star members can be derived from the properties of the cluster,” the astronomers stated.

“It, however, requires extensive monitoring on an as-long-as-possible time base line. This requirement may explain why not many clusters have been studied for their variability content so far, compared to the number of known and characterized clusters.”

These particular stars in NGC 3766, however, were puzzling.

“The stars are somewhat hotter and brighter than the Sun, but otherwise apparently unremarkable,” ESO stated, yet they had variations of about 0.1% of each star’s normal brightness.

Cepheid Variable Star.  Credit:  Hubble Space Telescope
Cepheid Variable Star. Credit: Hubble Space Telescope

It’s possible, but not proven yet, that perhaps the stars’ spin has something to do with the brightness.

Some of the observed objects whip around at speeds so fast that some material might be punted away from the star and into space, the astronomers wrote in a press release.

“In those conditions, the fast spin will have an important impact on their internal properties, but we are not able yet to adequately model their light variations,” stated Nami Mowlavi, another Geneva Observatory astronomer who led the paper.

Also, astronomers haven’t named this class of stars yet. Do you have any ideas? For more information and to generate suggestions, you can read the paper here in Astronomy & Astrophysics. Then you can leave your thoughts in the comments.

Source: European Southern Observatory

Another Ceasing Cepheid

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Earlier this year, I wrote an article about a Cepheid variable star named V19 in M31. This Cepheid was one that once pulsated strongly and was one of the variables Hubble first used to find the distance to the Andromeda galaxy. But today, V19 is a rare instance of a Cepheid that has seemingly, stopped pulsating. Another example of this phenomenon is that of Polaris, which has decreased in the amplitude of brightnesses by nearly an order of magnitude in the past century, although some reports indicate that it may be beginning to increase again. Meanwhile, a new paper is looking to add another star, HDE 344787, to this rare category and according to the paper, it may be “even more interesting than Polaris”.

The star in question, HDE 344787, is a F class supergiant. Although the variations in brightness have been difficult to observe, due to their small amplitude, astronomers have revealed two fundamental pulsation modes corresponding to 5.4 days and 3.8 days. But perhaps even more interesting, is that the 5.4 day period seems to be growing. Careful analysis of the data suggests that this period is growing by about 13 seconds per year. This finding is in strong agreement with what is predicted by models of stellar evolution for stars with metallicity similar to the sun passing through the instability strip for the first time.

HDE 344787 is similar in Polaris in that both stars share the same spectral type. However, the existence of two modes of pulsation is not seen in Polaris. The lengthening of the period of pulsation, however, is seen. For Polaris, its variation is growing by 4.5 seconds per year. Another similarity is that, like Polaris and V19, has been decreasing in the amplitudes of its brightness since at least 1890.

While the addition of this star to the collection of Cepheids that have decreased their amplitude, it does little to solve the mystery of why they might do so. Currently, both Polaris and HDE 344787 lie near the middle of the instability strip and, as such, are not simply evolving out of the region of instability. However, the confirmation of second pulsational mode may lend support to the notion that a change in one of these modes may serve to dampen the other, creating an effect known as the Blazhko Effect.

Ultimately, this star will require further observations to understand its nature better. Due the the faintness of this star (~10th magnitude) as well as the small change in brightness from the pulsations and the dense stellar field on which it lies, observations have been notoriously challenging.

The Little Cepheid that Stopped

Cepheid Variable Star. Credit: Hubble Space Telescope

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When Hubble first discovered a Cepheid variable in the galaxy M31, the universe grew. Previously, many astronomers had held that the fuzzy “spiral nebulae” were small patches of gas and dust within our own galaxy, but through the Period-Luminosity relationship which allowed him to determine the distance, Hubble demonstrated that these were “island universes”, or galaxies in their own right.

Soon after, Hubble (as well as other astronomers) began searching other fuzzy patches for Cepheids. Among them was the spiral galaxy M33 in which he discovered 35 Cepheids. Among them was V19 which had a 54.7 day period, an average magnitude of 19.59 ± 0.23 MB, and an amplitude of 1.1 magnitudes. But according to recent work revealed at the recent American Astronomical Society meeting, V19 no longer seems to be pulsating as a Cepheid.

The new research uses observations from the 3.5m Wisconsin, Indiana, Yale, and NOAO (WIYN) Observatory as well as the 1.3m Robotically Controlled Telescope (RCT) operated jointly by a group of universities and research institutions. The new observations confirm a 2001 report that found V19 had decreased its brightness amplitude to at least less than 10% of the magnitude reported by Hubble in 1926, and possibly further as any fluctuations were below the threshold detectable by the instruments.

Now, if any variation exists, it is less than 0.1 magnitudes. The new study reports that there may be some small fluctuations, but due to inherent uncertainty in the observations, it barely exceeds the background noise and the announcers did not commit to these findings. Instead, they pledged to continue observations with larger instruments to the equation to push down the instrumental error as well as adding spectroscopic measurements to investigate other changes in the star. Another of the peculiar changes V19 has undergone is an increase of about half of a magnitude to 19.08 ± 0.05.

These changes are strikingly similar to another, more famous star: Polaris. Due to its much closer nature, observations have been much more frequent and with lower detection thresholds. This star had previously been reported to have an amplitude of 0.1 magnitudes which, according to a 2004 study, had decreased to 0.03 magnitudes. Additionally, based on ancient records, astronomers have estimated that Polaris has also brightened about a full magnitude in the past 2,000 years.

According to Edward Guinan of Villanova University and one of the members of the new observational team, “both stars are experiencing unexpectedly fast and large changes in their pulsation properties and brightness that are not yet explained by theory.”

The primary explanation for this dramatic change is simple evolution: As the stars have aged, they have moved out of the instability strip, a region on the HR diagram in which stars are prone to pulsations. But these stars may not be entirely lost from the family of periodic variables. In 2008, a study led by Hans Bruntt of the University of Sidney suggested that Polaris’ amplitude may be increasing. The team found that from 2003 to 2006, the scale of the oscillations had increased by 30%.

This has led other astronomers to suspect that there may be an additional effect in play in Cepheids known as the Blazhko Effect. This effect, often seen in RR Lyrae stars (another type of periodic variables), is a periodic variation of the variation. While no firm explanation exists for this effect, astronomers have suggested that it may be due to multiple pulsational modes that interfere constructively and destructively and occasionally forming resonances.

Ultimately, these strange changes in brightness are unexplained and will require astronomers to have to carefully monitor these stars, as well as other Cepheids to search for causes.

Longstanding Cepheid Mass Mystery Finally Solved

An artist's impression of the binary system OGLE-LMC-CEP0227, which contains a classical Cepheid variable star. By measuring this system, astronomers were able to settle a longstanding debate over the masses of these types of stars. Image credit: ESO/L. Calçada

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Cepheid variable stars – a class of stars that vary in brightness over time – have long been used to help measure distances in our local region of the Universe. Since their discovery in 1784 by Edward Pigott, further refinements have been made about the relationship between the period of their variability and their luminosity, and Cepheids have been closely studied and monitored by professional and amateur astronomers.

But as predictable as their periodic pulsations have become, a key aspect of Cepheid variables has never been well-understood: their mass. Two different theories – stellar evolution and stellar pulsation – have given different answers as to the masses that these stars should be. What has long been needed to correct this error was a system of eclipsing binary stars that contained a Cepheid, so that the orbital calculations could yield the mass of the star to a high degree of accuracy. Such a system has finally been discovered, and the mass of the Cepheid it contains has been calculated to within 1%, effectively ending a discrepancy that has persisted since the 1960s.

The system, named OGLE-LMC-CEP0227, contains a classical Cepheid variable (as opposed to a Type II Cepheid, which is of lower mass and takes a different evolutionary track) that varies over 3.8 days. It is located in the Large Magellanic Cloud, and as the stars orbit each other over a period of 310 days, they eclipse each other from our perspective on Earth. It was detected as part of the Optical Gravitational Lensing Experiment, and you can see from the acronym soup that this yields the first part of the name, the Large Magellanic Cloud the second, and CEP stands for Cepheid.

A team of international astronomers headed by Grzegorz Pietrzynski of Universidad de Concepción, Chile and Obserwatorium Astronomiczne Uniwersytetu Warszawskiego, Poland measured the spectra of the system using the MIKE spectrograph at the 6.5-m Magellan Clay telescope at the Las Campanas Observatory in Chile and the HARPS spectrograph attached to the 3.6-m telescope of the European Southern Observatory at La Silla.

The team also measured the changes in brightness and slight red and blueshift of the light from the stars as they orbited each other, as well as the pulsing of the Cepheid. By taking all of these measurements, they were able to create a model of the masses of the stars that should yield the orbital mechanics of the system. In the end, the mass predicted by stellar pulsation theory agreed much more with the calculated mass than that predicted by stellar evolution theory. In other words, stellar pulsation theory FTW!!

They published their results today in a letter to Nature, and write in the conclusion of the letter: “The overestimation of Cepheid masses by stellar evolution theory may be the consequence of significant mass loss suffered by Cepheids during the pulsation phase of their lives – such loss could occur through radial motions and shocks in the atmosphere. The existence of mild internal core mixing in the main-sequence progenitor of the Cepheid, which would tend to decrease its evolutionary mass estimate, is another possible way to reconcile the evolutionary mass of Cepheids with their pulsation mass.”

Cepheid variables take their names from the star Delta Cephei (in the constellation Cepheus), which was discovered by John Goodricke to be a variable star a few months after Pigott’s discovery in 1784. There are many different types of variable stars, and if you are interested in learning more or even participating in observing and recording their variability, the American Association of Variable Star Observers has a wealth of information.

Source: ESO, original Nature letter