One of the most fundamental questions astronomers ask about an object is “What’s its distance?” For very faraway objects, they use classical Cepheid variable stars as “distance rulers”. Astronomers call these pulsating stars “standard candles”. Now there’s a whole team of them precisely clocking their speeds along our line of sight.
What makes a classical Cepheid a “standard candle” in the darkness of the Universe? It’s that pulsation. Not only does a Cepheid grow larger in a regular rhythm, but its brightness changes over predictable periods of time. In the early 1900s, astronomer Henrietta Leavitt studied thousands of these stars. She found something pretty interesting: there’s a strong relationship between a Cepheid’s luminosity and its pulsation period. And that’s a useful relationship.
When you compare a Cepheid’s luminosity to its pulsation period, you can derive the star’s distance. This relationship appears to be true for all known Cepheids. That’s why they’re considered an important part of the cosmic distance ladder. They’re the main benchmark for scaling the huge distances between galaxies and galaxy clusters.
Types of CepheidsThere are different “flavors” of Cepheids. The “classical” ones have pulsation periods ranging from a few days to a few months. They’re all more massive than the Sun and can be up to a hundred thousand times more luminous. Their radii can change pretty drastically during a cycle—some grow by millions of kilometers and then shrink. Type II Cepheids have pulsation periods between 1 and 50 days and are usually very old, low-mass stars. There are other types, including anomalous Cepheids with very short periods. Scientists also know about double-mode Cepheids with “heartbeats” that pulsate in two or more modes.
Some pretty well-known stars are Cepheid variables. For example, Polaris—the well-known “North Star” is one, as is RR Puppis, Delta Cephei, and Eta Aquilae—all visible from Earth. Why these stars pulsate is still being studied but here’s a very basic look at their process. The core of the star produces heat which heats the outer layers. They expand, and then cool. Radiation is escaping, which makes the star appear brighter. The cooler gas contracts under gravity and makes the star look smaller and cooler. Of course, the devil is in the details, which is why astronomers want to know more about the processes these stars undergo.
Polaris A (Pole Star) with its two stellar companions, Polaris Ab and Polaris B. Polaris itself is a Cepheid type variable star. Artists impression. Credit: NASAHowever, it turns out Cepheids are not exactly easy to study. For one thing, it’s tough to measure their pulsations and radial velocities accurately. In addition, some have companion stars and the presence of a nearby star complicates any measurements. For another thing, different instruments and measuring methods give slightly different results, which doesn’t help astronomers understand those stars any better.
Precision Measurements of Cepheid VariablesMeasuring the intricacies of Cepheid pulsations requires spectroscopic techniques that can measure light from stars and break it down into its component wavelengths. That reveals a lot of data about a star, including its chemical makeup, temperature, and motions in space.
Calibrated Period-luminosity Relationship for Cepheid variables. Courtesy Spitzer Space Telescope/IPAC.A worldwide consortium of astronomers led by Richard I. Anderson at Switzerland’s École Polytechnique Fédérale de Lausanne (EPFL) is measuring specific properties of classical and other Cepheids using two high-resolution spectrographs. One is called HERMES on La Palma in the northern hemisphere and the other is CORALIE in Chile. They both detected tiny shifts in the light of target Cepheids. Those shifts gave valuable information about the motions of the stars.
“Tracing Cepheid pulsations with high-definition velocimetry gives us insights into the structure of these stars and how they evolve,” he said. “In particular, measurements of the speed at which the stars expand and contract along the line of sight—so-called radial velocities—provide a crucial counterpart to precise brightness measurements from space. However, there has been an urgent need for high-quality radial velocities because they are expensive to collect and because few instruments are capable of collecting them.”
VELOCE is on the JobThe team’s measurement project is called the VELOCE Project—short for VELOcities of CEpheids. It’s 12-year-long collaboration among astronomers and astrophysicists. Anderson began the VELOCE project during his Ph.D work at the University of Geneva, continued it as a postdoc in the US and Germany, and has now completed it at EPFL.
According to Ph.D student Giordano Viviani, the data from the project are already enabling new discoveries about Cepheids. “The wonderful precision and long-term stability of the measurements have enabled interesting new insights into how Cepheids pulsate,” Viviani said. “The pulsations lead to changes in the line-of-sight velocity of up to 70 km/s, or about 250,000 km/h. We have measured these variations with a typical precision of 130 km/h (37 m/s), and in some cases as good as 7 km/h (2 m/s), which is roughly the speed of a fast walking human.”
Uncovering New Details about these Pulsating StarsThe VELOCE project’s precision measurements also revealed some strange facts about these stars. For example, there’s an interesting phenomenon called the Hertzsprung Progression. It describes double-peaked bumps in a Cepheid’s pulsations. Astronomers aren’t quite sure yet why these bumps occur. But, they could give some insight into the structure of Cepheid variables, particularly the so-called “classical” ones.
Other Cepheids show very complex variability, and changes in their radial velocities are not always consistent with predicted periods, according to postdoctoral researcher Henryka Netzel. “This suggests that there are more intricate processes occurring within these stars, such as interactions between different layers of the star, or additional (non-radial) pulsation signals that may present an opportunity to determine the structure of Cepheid stars by asteroseismology,” Netzel said.
As part of their study, the team also measured 77 Cepheids that are part of binary systems. One in three Cepheids “lives” in a binary system, and often those unseen companions are detectable by velocity measurements. Characterizing the different “flavors” of Cepheids and the intricacies of their pulsations has larger implications than determining their radial velocities and bumps in their periods, according to Anderson. “Understanding the nature and physics of Cepheids is important because they tell us about how stars evolve in general, and because we rely on them for determining distances and the expansion rate of the Universe,” Anderson said, noting that VELOCE is also providing a valuable “cross-check” with Gaia measurements. It’s on track to conduct a large-scale survey of Cepheid radial velocity measurements.
Cross-checking with GaiaAdditionally, VELOCE provides the best available cross-checks for similar, but less precise, measurements from the ESA mission Gaia. That spacecraft is on track to conduct the largest survey of Cepheid radial velocity measurements. Data from that mission provides a growing three-dimensional map of millions of stars in the Milky Way and beyond. It not only charts their positions but also their motions (including radial velocity), as well as temperatures and compositions. Combined with high-precision data from VELOCE about Cepheids, astronomers should soon be able to get a handle on stellar and galactic evolutionary history.
For More InformationHigh-precision Measurements Challenge the Understanding of Cepheids
VELOcities of CEpheids (VELOCE)
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Universe Today has had some incredible discussions with a wide array of scientists regarding impact craters, planetary surfaces, exoplanets, astrobiology, solar physics, comets, planetary atmospheres, planetary geophysics, cosmochemistry, meteorites, radio astronomy, extremophiles, organic chemistry, black holes, cryovolcanism, and planetary protection, and how these intriguing fields contribute to our understanding regarding our place in the cosmos.
Here, Universe Today discusses the mysterious field of dark matter with Dr. Shawn Westerdale, who is an assistant professor in the Department of Physics & Astronomy and head of the Dark Matter and Neutrino Lab at the University of California, Riverside, regarding the importance of studying dark matter, the benefits and challenges, how dark matter can teach us about finding life beyond Earth, the most exciting aspects about dark matter he’s studied throughout his career, and advice for upcoming students who wish to pursue studying dark matter. So, what is the importance of studying dark matter?
“About 80% of the mass of all matter in the universe is dark matter, despite the fact that our (otherwise extremely successful) model of fundamental particle physics cannot explain what it is,” Dr. Westerdale tells Universe Today. “We can see the gravitational influence of dark matter in our own galaxy and throughout the entire structure of the observable universe. It leaves a clear imprint on all of our cosmological and astrophysical observations through these gravitational interactions, so we know it is there and it does a remarkable job of explaining what we see. But we have no idea what it actually is made of, and this is an essential part of understanding nature.”
The term “dark matter” was first coined in 1906 by French mathematician and theoretical physicist, Dr. Henri Poincaré, to describe work from 1884 by the British mathematical physicist, Dr. William Thomson (Lord Kelvin), regarding velocities of stars and some potentially being dark bodies. Throughout the rest of the 20th century, dark matter became a focal point in hypothesizing the behavior of galaxies and galaxy clusters with countless studies being published from academia, including the California Institute of Technology, along with research organizations like the SETI Institute. Despite decades of research, including the hypothesis of “cold”, “warm”, and “hot” dark matter, this mysterious substance has yet to be observed. Therefore, what are some of the benefits and challenges of studying dark matter?
Dr. Westerdale tells Universe Today, “We haven’t found it yet, but we have ruled out many models, and in doing so we have helped refine our understanding of nature by ruling out possible modifications to the Standard Model of particle physics. On a sociological level, the study of dark matter has led to many new technologies for detecting radiation. Some of these may lead to new quantum technologies, and others are being developed into new medical imaging devices, just to name a few examples.”
The three methods for attempting to observe dark matter include direct detection, indirect detection, and laboratory experiments using a myriad of laboratories from several countries around the world, including the Large Hadron Collider, which is the world’s largest particle collider. Additionally, several ground- and space-based telescopes have conducted surveys to try and create dark matter maps, including NASA’s Hubble Space Telescope, the Canada-France-Hawaii Telescope, the VLT Survey Telescope, and the Subaru Telescope. But what are the most exciting aspects about dark matter that Dr. Westerdale has studied during his career?
Dr. Westerdale tells Universe Today, “To me the most exciting aspect of dark matter research has been the magnitude of the question. We have such successful models of cosmology and particle physics, and yet for all the success of these models, we still don’t know what most of the universe is even made of or how it got here!”
The study of dark matter comprises some of the most fundamental questions pertaining to cosmology, the nature of the universe, and our place in it. What is the universe made of? How did it form? How did galaxies form? How do galaxies behave the way they do? How has all of this led to us being here and writing articles about dark matter like this one? The answers to these questions continue to elude astrophysicists, cosmologists, and countless other scientists despite decades of research, experiments, models, and hypotheses.
Dr. Westerdale tells Universe Today, “One of the fun challenges of dark matter detection is that we are looking for extremely rare interactions and so we have to go to extraordinary lengths to make our experiments as quiet as possible. We put our detectors in deep underground labs, up to a mile underground, to avoid noise from cosmic rays, and levels of radioactivity that are normally so low they cannot be measured can swamp the signals we’re looking for. It is an exciting challenge to confront these things in our research and figure out how to design detectors that can meet all of our goals.”
Despite the lack of observing dark matter and confirming its existence, this nonetheless signals that the next generation of dark matter enthusiasts, whether they become astrophysicists, cosmologists, or come from other scientific backgrounds, will have their work cut out for them, with some possibly being the ones to confirm dark matter’s existence. Like nearly all scientific research trajectories, the study of dark matter involves constant collaboration between scientists from a myriad of backgrounds and expertise’s. Therefore, what advice can Dr. Westerdale offer to upcoming students who wish to pursue studying dark matter?
Dr. Westerdale tells Universe Today, “Experimental dark matter physics requires a very large breadth of knowledge, and so don’t silo your studies — any physics, math, and engineering skills you learn will at some point be useful. Programming skills are especially important, as are learning statistics, chemistry, and other engineering skills. And when you encounter something new, take the time to learn how it works on a fundamental level — it will be worth it later on once you can see how it fits into the big picture.”
Will we ever observe dark matter and how will it help us better understand our place in the universe in the coming years and decades? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!
The post Dark Matter: Why study it? What makes it so fascinating? appeared first on Universe Today.
One of the easiest ways to find exoplanets is using the transit method. It relies upon monitoring the brightness of a star which will then dim as a planet passes in front of it. It is of course possible that other objects could pass between us and a star; perhaps binary planets, tidally distorted planets, exocomets and, ready for it…. alien megastructures! A transit simulator has been created by a team of researchers and it can predict the brightness change from different transiting objects, even Dyson Swarms in construction.
51 Pegasi-b was the first exoplanet discovered in 1995 and it sparked the development of numerous ground-based and space-based instruments. The launch of the Kepler Space Telescope and the Transiting Exoplanet Survey Satellite (TESS) in 2018 popularised the transit method, leading to the discovery of over 4,000 exoplanets. As instruments have become increasingly sensitive and precise, research has progressed from simply detecting exoplanets to studying their detailed characteristics.
Illustration of NASA’s Transiting Exoplanet Survey Satellite. Credit: NASA’s Goddard Space Flight CenterTransit photometry has uncovered signatures of many interesting phenomena beyond the detection of exoplanets and eclipsing binaries. This technique has been instrumental in identifying features such as star-spots, and signatures of tidal interactions between host stars and exoplanets leading to significant growth in the sub-field of Asteroseismology
The study of transiting exoplanets and their timing variations has led to many discoveries. Non-transiting planets in distant solar systems have been found, orbital decay, disintegrating planets, exocomets and exomoon candidates has all been identified. Additionally, and perhaps of particular interest is that transit photometry has detected signals that have sparked interest in the search for technosignatures for the evidence of advanced civilizations.
It is important to note that no technosignatures have been confirmed yet but such signatures would not arise form natural processes and would demonstrate the presence of intelligent life. The signatures would come from a wide range of astroengineering projects like Dyson Spheres (a theoretical shell surrounding a star to capture its energy output) or the newly conceptualised Dyson Swarms (habitable satellites and energy collectors that orbit the star in formation.
The research team led by Ushasi Bhowmick from the Indian based Space Application Centre has reported that they have developed a transit simulator that can not only generate light curves for exoplanets but also for any object of any size or shape! The simulation uses the Monte-Carlo technique that predicts all possible outcomes of an uncertain event. In this instance it can predict the light curve when an object of any shape or size transits across the disk of star.
Artist’s impressions of two exoplanets in the TRAPPIST-1 system (TRAPPIST-1d and TRAPPIST-1f). Credit: NASA/JPL-CaltechWhen the simulation was tested against actual exoplanet systems such as Trappist-1 it nicely predicted the light curve. It can also be used to model tidal distortions in binary star systems and even predict the light curve of non-natural objects such as the alien megastructures. The simulator has shown itself to be an invaluable method for understanding a wide range of transit phenomena.
Source : A General-Purpose Transit Simulator for Arbitrary Shaped Objects Orbiting Stars
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