Why do astronomers use supernova to measure distance in space?

Why do astronomers use supernova to measure distance in space?

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Why can't the distances between our Sun and the celestial objects be measured directly and instead had to rely on looking around for supernova? Is these events frequent, evenly distributed and lasting enough for astronomers to measure any place in our observable universe? Is this method of measurement more preferable than others currently?does this method had any limitations such as an absent of supernova nearby?

Type Ia supernovae are not common; they are quite rare events, happening maybe once per 100-200 years in a big galaxy. Nevertheless they have two properties that make them fantastically useful for distance measurement.

  1. They are (very close to) "standard candles". The physics of the supernova detonation, thought to be when a white dwarf accretes matter and exceeds the Chandrasekhar limit, is very "standardised". The bomb goes off in exactly the same way with the same amount of identical explosive. That means to a good approximation, measuring the apparent brightness of a type Ia supernova and comparing it with nearby examples means that the distances to these events can be accurately estimated.

  2. They are really luminous and last 2-3 weeks. This means that they can be seen at enormous distances, they almost outshine the galaxies that they are in, and they last long enough for astronomers to discover them in automated surveys and still have time to follow them up and measure their light curves and estimate their peak brightness.

The limitations are that you can't choose which galaxies you measure the distance to. You have to wait for a supernova to go off and that might take 100-200 years or more for a particular galaxy. There are also continuing debates about just how standard a candle these objects are. It is possible that more distant galaxies that we see in the very early universe made stars with a different composition (far less elements heavier than helium) and that this alters things a bit. Other problems are associated with "de-reddening" the supernova light curves to account for the possibility of obscuring dust in the host galaxy. It is also possible that there is more than one way to produce a type Ia supernova (accretion of additional mass or merger of two white dwarfs).

I am puzzled by your question "Why can't the distances between our Sun and the celestial objects be measured directly?". Galaxies are much too far away to have a trigonometric parallax distance. Methods like using Cepheids or RR Lyrae variables don't work for very distant galaxies because we can't resolve the individual stars. You could estimate a distance using a measurement of redshift and a cosmological model for the expansion of the Universe, but really the point of using these supernovae was and is to test and improve the cosmological models.

Physics KS3/KS4: Professor Brian Cox - Why do astronomers use 'light years' to measure distance in space?

When astronomers talk about the distance to the stars and galaxies, they don't ever talk in metres or kilometres.

And that's just because the distances are so great that weɽ have to keep saying millions and billions and trillions, and scientists are as lazy as everybody else so they don't want to do that.

And so we measure distances in 'light years', which is very simply the distance that light travels in one year.

Now, light travels very fast - 300,000 kilometres per second - so, to get the number of kilometres in a light year, youɽ have to take the 300,000 kilometres per second and multiply it by the number of seconds in a year.

Now, if you do that, you get approximately 9.4 million million kilometres.

So a light year is just under 10 million million kilometres.

This video discusses measurements in astronomy and explains why astronomers do not use standard forms like metres or kilometres to measure distance, but instead refer to light years.

The definition is given as the distance light travels in one year, and the scale of magnitude is highlighted by converting a light year into kilometres to demonstrate that light years simply make the numbers more manageable.

Teacher notes

Points for discussion

  • A common challenge with space physics is the proportions involved. Very large (and very small) entities can be challenging for students to grasp as they are so hard to visualise. When taking about the distances from the Earth to the sun, or distances to other stars, the measurements are vast. It is really useful for students to appreciate that light years are used as a unit of measurement simply to make the numbers more manageable.

Suggested activities

For KS3, after watching the film, allow students to practise writing out the distance of a light year in kilometres so that they can experience the potential difficulty in dealing with such large numbers.

Students could also practise calculating other astronomical proportions in kilometres to further emphasise the point.

The film could also provide an opportunity to practise using standard form, if at an appropriate stage of the curriculum. Calculating using light years instead of standard form could then be done to demonstrate how using light years simplifies the process.

Any models or visualisations that can be used to demonstrate the relative distances may be useful to support an awareness of order of magnitude and scale.

Curriculum information

Suitable for KS3 England, Wales and NI and CfE Scotland, S1-3.

Why do astronomers use supernova to measure distance in space? - Astronomy

An astronomical unit is like any other unit: completely arbitrary, but useful. Why do we use a foot? Or a meter? Or an inch? Or a gallon? It's just a useful way of thinking of the sizes of things. So, we have units at all different sizes. It wouldn't make much sense to measure the distance from here to Europe in inches (so we use miles), in the same way it wouldn't be useful to measure the size of an atom using inches (we use a unit called Angstroms Angstroms , which are 0.0000000001 meters). Astronomical units are just a useful way to think about the solar system relative to the distance from Earth to the Sun, because it's easy to use.

For an example of how easy it is, you could either say:
- The Earth is 93000000 miles from the sun, Saturn is 890700000 miles from the sun.
- The Earth is 1AU from the sun, Saturn is 9.6AU from the sun.

When you use AU, it is easier to understand the relative distances, and that Saturn is about ten times farther from the sun.

The solar system is enormous, and interstellar space is even bigger. One astronomical unit is equal to 150 million kilometers. This makes it much easier to count the distances if they're in counts of Astronomic Units instead of having to count everything in millions or billions of kilometers.

Ideas, Inventions And Innovations

When NASA’s Transiting Exoplanet Survey Satellite launched into space in April 2018, it did so with a specific goal: to search the universe for new planets.

But in recently published research, a team of astronomers at The Ohio State University showed that the survey, nicknamed TESS, could also be used to monitor a particular type of supernova, giving scientists more clues about what causes white dwarf stars to explode—and about the elements those explosions leave behind.

“We have known for years that these stars explode, but we have terrible ideas of why they explode,” said Patrick Vallely, lead author of the study and an Ohio State astronomy graduate student. “The big thing here is that we are able to show that this supernova isn’t consistent with having a white dwarf (take mass) directly from a standard star companion and explode into it—the kind of standard idea that had led to people trying to find hydrogen signatures in the first place. That is, because the TESS light curve doesn’t show any evidence of the explosion slamming into the surface of a companion, and because the hydrogen signatures in the SALT spectra don’t evolve like the other elements, we can rule out that standard model.”

Their research, detailed in the Monthly Notices of the Royal Astronomical Society,represents the first published findings about a supernova observed using TESS, and add new insights to long-held theories about the elements left behind after a white dwarf star explodes into a supernova.

Those elements have long troubled astronomers.

A white dwarf explodes into a specific type of supernova, a 1a, after gathering mass from a nearby companion star and growing too big to remain stable, astronomers believe. But if that is true, then the explosion should, astronomers have theorized, leave behind trace elements of hydrogen, a crucial building block of stars and the entire universe. (White dwarf stars, by their nature, have already burned through their own hydrogen and so would not be a source of hydrogen in a supernova.)

But until this TESS-based observation of a supernova, astronomers had never seen those hydrogen traces in the explosion’s aftermath: This supernova is the first of its type in which astronomers have measured hydrogen. That hydrogen, first reported by a team from the Observatories of the Carnegie Institution for Science, could change the nature of what astronomers know about white dwarf supernovae.

“The most interesting thing about this particular supernova is the hydrogen we saw in its spectra (the elements the explosion leaves behind),” Vallely said. “We’ve been looking for hydrogen and helium in the spectra of this type of supernova for years—those elements help us understand what caused the supernova in the first place.”

Patrick Vallely

Credit: OSU

The hydrogen could mean that the white dwarf consumed a nearby star. In that scenario, the second star would be a normal star in the middle of its lifespan—not a second white dwarf. But when astronomers measured the light curve from this supernova, the curve indicated that the second star was in fact a second white dwarf. So where did the hydrogen come from?

Professor of Astronomy Kris Stanek, Vallely’s adviser at Ohio State and a co-author on this paper, said it is possible that the hydrogen came from a companion star—a standard, regular star—but he thinks it is more likely that the hydrogen came from a third star that happened to be near the exploding white dwarf and was consumed in the supernova by chance.

Kris Stanek

Credit: OSU

“We would think that because we see this hydrogen, it means that the white dwarf consumed a second star and exploded, but based on the light curve we saw from this supernova, that might not be true,” Stanek said.

“Based on the light curve, the most likely thing that happened, we think, is that the hydrogen might be coming from a third star in the system,” Stanek added. “So the prevailing scenario, at least at Ohio State right now, is that the way to make a Type Ia (pronounced 1-A) supernova is by having two white dwarf stars interacting—colliding even. But also having a third star that provides the hydrogen.”

For the Ohio State research, Vallely, Stanek and a team of astronomers from around the world combined data from TESS, a 10-centimeter-diameter telescope, with data from the All-Sky Automated Survey for Supernovae (ASAS-SN for short.) ASAS-SN is led by Ohio State and is made up of small telescopes around the world watching the sky for supernovae in far-away galaxies.

TESS, by comparison, is designed to search the skies for planets in our nearby galaxy—and to provide data much more quickly than previous satellite telescopes. That means that the Ohio State team was able to use data from TESS to see what was happening around the supernova in the first moments after it exploded—an unprecedented opportunity.

The team combined data from TESS and ASAS-SN with data from the South African Large Telescope to evaluate the elements left behind in the supernova’s wake. They found both hydrogen and helium there, two indicators that the exploding star had somehow consumed a nearby companion star.

“What is really cool about these results is, when we combine the data, we can learn new things,” Stanek said. “And this supernova is the first exciting case of that synergy.”

The supernova this team observed was a Type Ia, a type of supernova that can occur when two stars orbit one another—what astronomers call a binary system. In some cases of a Type I supernova, one of those stars is a white dwarf.

A white dwarf has burned off all its nuclear fuel, leaving behind only a very hot core. (White dwarf temperatures exceed 100,000 degrees Kelvin—nearly 200,000 degrees Fahrenheit.) Unless the star grows bigger by stealing bits of energy and matter from a nearby star, the white dwarf spends the next billion years cooling down before turning into a lump of black carbon.

But if the white dwarf and another star are in a binary system, the white dwarf slowly takes mass from the other star until, eventually, the white dwarf explodes into a supernova.

Type I supernovae are important for space science—they help astronomers measure distance in space, and help them calculate how quickly the universe is expanding (a discovery so important that it won the Nobel Prize in Physics in 2011.)

“These are the most famous type of supernova—they led to dark energy being discovered in the 1990s,” Vallely said. “They are responsible for the existence of so many elements in the universe. But we don’t really understand the physics behind them that well. And that’s what I really like about combining TESS and ASAS-SN here, that we can build up this data and use it to figure out a little more about these supernovae.”

Scientists broadly agree that the companion star leads to a white dwarf supernova, but the mechanism of that explosion, and the makeup of the companion star, are less clear.

This finding, Stanek said, provides some evidence that the companion star in this type of supernova is likely another white dwarf.

“We are seeing something new in this data, and it helps our understanding of the Ia supernova phenomenon,” he said. “And we can explain this all in terms of the scenarios we already have—we just need to allow for the third star in this case to be the source of the hydrogen.”

ASAS-SN is supported by Las Cumbres Observatory and funded in part by the Gordon and Betty Moore Foundation, the National Science Foundation, the Mt. Cuba Astronomical Foundation, the Center for Cosmology and AstroParticle Physics at Ohio State, the Chinese Academy of Sciences South American Center for Astronomy and the Villum Fonden in Denmark.

Contacts and sources:
Laura Arenschield
Ohio State University

Citation: ASASSN-18tb: a most unusual Type Ia supernova observed by TESS and SALT.
P J Vallely, M Fausnaugh, S W Jha, M A Tucker, Y Eweis, B J Shappee, C S Kochanek, K Z Stanek, Ping Chen, Subo Dong, J L Prieto, T Sukhbold, Todd A Thompson, J Brimacombe, M D Stritzinger, T W-S Holoien, D A H Buckley, M Gromadzki, Subhash Bose. Monthly Notices of the Royal Astronomical Society, 2019 487 (2): 2372 DOI: 10.1093/mnras/stz1445

Why do astronomers use supernova to measure distance in space? - Astronomy

I have recently heard that there is research or data that would suggest that the speed of the galaxies at the edge of the Universe are actually increasing with speed as they continue. Do you have any more information on this?

Actually, these results show that the expansion of space is speeding up. Since space expands everywhere, this "speeding up" doesn't just happen to galaxies on the 'edge' of the Universe, but to every part of the Universe.

Because the expansion is a rate (like a velocity), then a changing expansion is a changing rate, or an acceleration. We therefore say that the Universe is undergoing accelerated expansion. These results actually started coming in in the late 1990s and the early 2000s, so they're new but not brand new. The projects that studied them were called The Supernova Cosmology Project, the High-Z Supernova Search Team, and the Supernova Legacy Survey.

You may have heard of dark energy, and this is in fact the same thing: the evidence for dark energy is that the universe is accelerating. Scientists determined this by using something called a 'standard candle.' A standard candle is an astrophysical object that has some characteristic that allows us to determine its total luminosity, even though it's very far away. Since the amount of that luminosity which we receive has to do with the distance to the object, standard candles can be used to figure out how far away an object is. You could conceivably do an experiment to prove this to yourself: if you know that a 60 watt lightbulb gives off a certain amount of energy, and then measure the energy received from a 60 watt lightbulb across the room from you, you could calculate the distance to that lightbulb.

Astronomers can take advantage of standard candles to determine the distance to objects like galaxies, and that's what happened in the projects I just mentioned. Using a type of supernova called a type Ia supernova, astronomers determined both the distance of the galaxy and the redshift of the galaxy. "Redshift" basically told them how much the Universe had expanded since the light left the supernova. The astronomers could then compare distance to expansion, and create a kind of 'expansion history' of the Universe.

It turned out that these results showed that the rate of the Universe's expansion was increasing!

Why does this lead us to the idea of 'dark energy'? Well, the increased rate of expansion means that the Universe is getting bigger and bigger. Since gravity is an attractive force, you would instead expect the Universe to want to get smaller. Gravity should 'pull' the Universe back together again. If the Universe continues to expand, faster and faster, some force or pressure must be 'pushing' it back out. This is the so-called dark energy. Dark energy is consistent with a host of other observations, so the supernova data is actually very, very cool because it gives a different kind of 'check' on the conclusions other teams have drawn.

This page was last updated June 28, 2015.

About the Author

Ann Martin

Ann finished her PhD at Cornell in May 2011, and has been a Curious volunteer since 2006. For her dissertation, she studied the distribution of hydrogen-rich galaxies in the nearby Universe using data from the Arecibo Observatory. Since then, she has been working on science education and public outreach projects for NASA Langley Research Center in Hampton, VA.

Our Work

Center for Astrophysics | Harvard & Smithsonian scientists refine extragalactic measurements in various ways:

Mapping the structure of the universe by measuring the distances to galaxies. CfA astronomers pioneered this method, from the first redshift survey initiated in 1977 through recent projects like the Two Micron All-Sky Survey (2MASS) Redshift Survey, completed in 2011.
Astronomers Unveil Most Complete 3-D Map of Local Universe

Surveying galaxies using the Baryon Oscillation Spectroscopic Survey (BOSS) to establish the “standard ruler” for extragalactic distances. This ongoing project has mapped tens of thousands of galaxies, and has provided the best measurements astronomers have of the acceleration of the expansion of the universe.
A One-Percent Measure of Galaxies Half the Universe Away

An optical and X-ray image of Kepler's supernova, a type Ia supernova described by Johannes Kepler in 1604. Type Ia supernovas are the explosions of white dwarfs, which astronomers use to measure extragalactic distances and the expansion rate of the universe.

Finding Distances to Type Ia Supernovae

Type Ia supernovae are known as “standard candles” due to their consistency, allowing us to measure distances based on their brightness. But what if these explosions aren’t quite as consistent as we thought? Due scientific diligence requires careful checks, so a recent study investigates whether the metallicity of a supernova’s environment affects the peak luminosity of the explosion.

Metallicity Dependence?

Type Ia supernovae are incredibly powerful tools for determining distances in our universe. Because these supernovae are formed by white dwarfs that explode when they reach a uniform accreted mass, the supernova peak luminosity is thought to be very consistent. This consistency allows these supernovae to be used as standard candles to measure distances to their host galaxies.

But what if that peak luminosity is affected by a factor that we haven’t taken into account? Theorists have proposed that the luminosities of Type Ia supernovae might depend on the metallicity of their environments — with high-metallicity environments suppressing supernova luminosities. If this is true, then we could be systematically mis-measuring cosmological distances using these supernovae.

Testing Abundances

Supernova brightnesses vs. the metallicity of their environments. Low-metallicity supernovae (blue shading) and high-metallicity supernovae (red shading) have an average magnitude difference of

0.14. [Adapted from Moreno-Raya et al. 2016]

Moreno-Raya and collaborators used spectra from the 4.2-m William Herschel Telescope to estimate oxygen abundances in the region where each of these supernovae exploded. They then used these measurements to determine if metallicity of the local region affects the luminosity of the supernova.

Determining Distances

The authors find that there are indeed differences in peak supernova luminosity based on metallicity of the local environment. Their observations support a trend in which more metal-rich galaxies host less luminous supernovae, whereas lower-metallicity galaxies host supernovae with greater luminosities — consistent with theoretical predictions.

This observational confirmation suggests that the metallicity of the progenitor may well play a role in peak supernova luminosity and, as a result, the distances at which we estimate they exploded. This systematic effect can, however, be easily corrected for in the distance-estimate procedure.

As the number of known supernovae is expected to drastically increase with the start of future large surveys such as the Large Synoptic Survey Telescope (LSST) or the Dark Energy Survey (DES), supernova distance measurements will soon be dominated by systematic errors rather than statistical ones. Correctly accounting for effects such as this apparent metallicity-dependence of supernovae continues to be important for accurately determining distances using Type Ia supernovae as indicators.


Manuel E. Moreno-Raya et al 2016 ApJ 818 L19. doi:10.3847/2041-8205/818/1/L19

Astronomers identify the best supernovae for measuring cosmic distances

Type Ia supernovae, which occur when burnt-out stars called white dwarfs detonate, have been used for years to help measure the distances to galaxies and the acceleration of our universe. But the tools aren't perfect, so researchers are analyzing the sites of the explosions to learn more about them and improve cosmic measuring tools. Using data from NASA's Galaxy Evolution Explorer, or GALEX, astronomers were able to show that a fraction of the Type Ia explosion sites they looked at are associated with hot young stars. This means that those areas are young in general, and that the explosions happened when relatively young white dwarf stars exploded. (Six of those youthful supernova sites are circled.) The astronomers then went on to show that these particular explosions occurred in more consistent ways, and thus are better standard tools for cosmology. In the future, other "top-of-the-line" Type Ia tools -- the ones associated with young stars -- can be used to measure the distances of galaxies out to six billion light-years away or farther. Credit: SDSS

The brilliant explosions of dead stars have been used for years to illuminate the far-flung reaches of our cosmos. The explosions, called Type Ia supernovae, allow astronomers to measure the distances to galaxies and measure the ever-increasing rate at which our universe is stretching apart.

But these tools aren't perfect. In the cosmic hardware store of our universe, improvements are ongoing. In a new report, appearing March 27 in the journal Science, astronomers identify the best, top-of-the-line Type Ia supernovae for measuring cosmic distances, pushing other, more clunky tools to the back of the shelf.

Using archived data from NASA's Galaxy Evolution Explorer (GALEX), scientists show that a particular class of Type Ia supernovae that occur near youthful stars can improve these measurements with a precision of more than two times that achieved before.

"We have discovered a population of Type Ia supernovae whose light output depends very precisely on how quickly they fade, making it possible to measure very exact distances to them," said Patrick Kelly of the University of California, Berkeley, lead author of the new study. "These supernovae are found close to populations of bright, hot young stars."

The findings will help light the way to understanding dark energy, one of the greatest mysteries in the field of cosmology, the study of the origin and development of the universe. Dark energy is the leading culprit behind the baffling acceleration of our cosmos, a phenomenon discovered in 1998. The acceleration was uncovered when astronomers observed that galaxies are pulling away from each other at increasing speeds.

The key to measuring this acceleration—and thus the nature of dark energy—lies with Type Ia supernovae, which work much like light bulbs strung across space. Imagine lining up 60-watt light bulbs across a field and standing at one end. The farthest light bulb wouldn't appear as bright as the closest one due to its distance. Since you know how bright the light bulb inherently is, you can use the extent of its dimming to figure out the distance.

Type Ia supernovae, also referred to as "standard candles," work in a similar way because they consistently shine with about the same amount of light. While the process that leads to these explosions is still not clear, they occur when the burnt-out core of a star, called a white dwarf, blasts apart in a regular way, briefly lighting up the host galaxy.

However, the explosions aren't always precisely uniform. They can differ considerably depending on various factors, which appear to be connected to the environments and histories of the exploding stars. It's as if our 60-watt bulbs sometimes give off 55 watts of light, skewing distance measurements.

Kelly and his team investigated the reliability of these tools by analyzing the surroundings of nearly 100 previous Type Ia explosions. They used data from GALEX, which detects ultraviolet light. Populations of hot, young stars in galaxies will shine brightly with ultraviolet light, so GALEX can distinguish between young and older star-forming communities.

The results showed that the Type Ia supernovae affiliated with the hot, young stars were significantly more reliable at indicating distances than their counterparts.

"These explosions are likely the result of youthful white dwarfs," said Kelly.

By focusing on this particular brand of Type Ia tools, astronomers will be able to, in the future, make even sharper measurements of the size and scale of our universe. According to the science team, this class of tools could work at distances up to six billion light-years away, and perhaps farther.

"GALEX surveyed the entire sky, allowing past and future eruptions of these high-quality standard candles to be identified easily," said Don Neill, a member of the GALEX team at the California Institute of Technology in Pasadena, not affiliated with the study. "Any improvement in the standard candles will have a direct impact on theories of dark energy, allowing us to home in on this mysterious force propelling the acceleration of the universe."

Caltech led the Galaxy Evolution Explorer mission and was responsible for science operations and data analysis. The mission ended in 2013 after more than a decade of scanning the skies in ultraviolet light. NASA's Jet Propulsion Laboratory in Pasadena, California, managed the mission and built the science instrument. The mission was developed under NASA's Explorers Program managed by the Goddard Space Flight Center, Greenbelt, Maryland. Researchers sponsored by Yonsei University in South Korea and the Centre National d'Etudes Spatiales (CNES) in France collaborated on this mission.

Astronomers Use Light Echos to Measure the Distance to a Star

Because stars are just points of light in the sky, it’s very difficult to know how far away they are. Astronomers use several techniques to measure distance, but they’ve got a new one now. By measuring echos of light bouncing off a distant nebula, researchers have fine-tuned their accuracy to an amazing level of precision.

Astronomers used ESO’s New Technology Telescope at La Silla to perform detailed observations of a star called RS Pup. It’s a member of a group of pulsating stars known as Cepheid variables. RS Pup changes in brightness by a factor of 5 every 41.4 days. It’s 10 times more massive than the Sun, 200 times larger and puts out 15,000 times more light.

You can look through these books and instructional materials from for more information about stars.

Because Cepheids pulse at a rate in proportion with their size, astronomers can measure how far they are by how often they pulsate.

But this only tells you how far they are relative to one another. So astronomers use a different technique called parallax to measure distance as well. If you want more info on this, check an episode of Astronomy Cast where we measure different techniques to measure distance in the Universe.

Now astronomers have come up with a second technique to measure distance to a star like RS Pup to confirm that the Cepheid variable technique is correct.

They did this by watching how light moves through the nebula of material shed by RS Pup in the past. Since light is going 300,000 km/s, it takes time to pass by various blobs of gas and dust in the nebula.

The researchers calculated the light curve from an event on the star, and then watched as that same curve passed different parts of the nebula. It was then a relatively straightforward calculation to determine how far away RS Pup is.

To really appreciate what’s happening, check out the video, where you can see pulses of light move through the nebula. I’ll warn you, it’s a 3.4 MB download.

According to their calculations, the star is 6,500 light-years away, give or take about 90 light years. It’s the most accurate distance to a Cepheid ever captured, with a 1% level of precision.

What’s Left Behind

Supernova explosions are dramatic, but the leftovers are just as interesting from a scientific point of view. These supernova remnants — including the Crab Nebula — contain information about the original system that exploded. They are also hotbeds of activity, containing powerful magnetic fields and hot plasma that can create shock waves in the surrounding material. As a result, supernova remnants are extremely important for understanding the life cycle of stars and physical processes in extreme environments.

With Type Ia supernovas, the exploding stars are completely destroyed. In the case of core-collapse supernovas, however, the remnant also harbors the neutron star or black hole created from the core of the dead star. For example, the Crab Nebula harbors a pulsar, a spinning neutron star that interacts with materials in the supernova remnant. In particular, it creates a disk of hot matter around it and a powerful jet shooting away, which heats up matter around it.