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Star that changed the universe

Andromeda Galaxy with insets of star V1

Observations of a star in the Andromeda Galaxy that changes its brightness in a regular pattern, convinced astronomers that our cosmos was huge. Edwin Hubble's further studies of such stars showed that the universe is expanding.

THOUGH THE UNIVERSE IS FILLED with billions upon billions of stars, observations of a single star in 1923 altered the course of modern astronomy. And, at least one famous astronomer of the time lamented that the discovery had shattered his worldview.

The star goes by the inauspicious name of Hubble variable number one, or V1, and resides two million light-years away in the outer regions of the Andromeda Galaxy. V1 belongs to a special class of pulsating star called Cepheid variables, which can be used to make reliable measurements of large cosmic distances.

The star helped Edwin Hubble show that Andromeda lies beyond our galaxy. Prior to the discovery of V1 many astronomers, including Harlow Shapley, thought ‘spiral nebulae’, such as Andromeda, were part of our Milky Way Galaxy.

Others weren’t so sure. In fact, Shapley and Heber Curtis held a public debate in 1920 over the nature of these nebulae. But it took Edwin Hubble’s discovery just a few years later to settle the debate.

Hubble sent a letter, along with a light curve of V1, to Shapley telling him of his discovery. After reading the note, Shapley reportedly told a colleague, “Here is the letter that destroyed my universe.”

The universe became a much bigger place after Edwin Hubble’s discovery.

Andromeda Galaxy with an overlay of a Cepheid star light curve

Cepheid variable stars like V1 change their brightness with a regular pattern. This characteristic enables astronomers to use them to measure distances in the cosmos, by comparing their apparent brightness with their calculated theoretical brightness. Courtesy NASA, ESA, and Z. Levay (STScI), HHT (STScI/AURA), AAVSO. Acknowledgment: T. Rector (University of Alaska, Anchorage).

Cosmic distance ladder

In commemoration of this landmark observation, astronomers with the Space Telescope Science Institute’s Hubble Heritage Project partnered with the American Association of Variable Star Observers (AAVSO) to study the star.

AAVSO observers followed V1 for six months, producing a plot, or light curve, of the rhythmic rise and fall of the star’s light. Based on this data, the Hubble Heritage team scheduled Hubble Space Telescope time to capture Wide Field Camera 3 images of the star at its dimmest and brightest light levels.

“This observation is a reminder that Cepheid variables are still relevant today,” explains Max Mutchler of the Heritage team. “Astronomers are using them to measure distances to galaxies much farther away than Andromeda. They are the first rung on what astronomers call the cosmic distance ladder.”

Edwin Hubble's original photo of Andromeda

Edwin Hubble's original photo of Andromeda, showing three stars of interest marked 'N'. The one at the top became even more interesting when it was recognised as being variable (hence 'VAR'). This is Hubble's V1 star.

Copies of the photograph Edwin Hubble made in 1923 flew onboard space shuttle Discovery in 1990 on the mission that deployed Hubble. Two of the remaining five copies were part of space shuttle Atlantis’s cargo in 2009 for NASA’s fifth servicing mission to Hubble.

The most important star

Edwin Hubble’s observations of V1 became the critical first step in uncovering a larger, grander universe. He went on to measure the distances to many galaxies beyond the Milky Way by finding Cepheid variables within them. The velocities of those galaxies, in turn, allowed him to determine that the universe is expanding.

“V1 is the most important star in the history of cosmology,” says astronomer Dave Soderblom of the Space Telescope Science Institute, who proposed the V1 observations.

The space telescope that bears his name continues using Cepheids to refine the expansion rate of the universe and probe galaxies that were far beyond Edwin Hubble’s reach.

Adapted from information issued by STScI. Images courtesy NASA, ESA, and the Hubble Heritage Team (STScI/AURA). Acknowledgment: R. Gendler.

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Hubble bursts dark energy bubble

Galaxy NGC 5584

Galaxy NGC 5584 was one of eight galaxies astronomers studied to measure the universe's expansion rate. Two special kinds of stars—Type Ia supernovae and Cepheid variable stars—were used as "cosmic yardsticks", due to their predictable brightnesses.

  • Our universe seems to be expanding faster and faster with time
  • ‘Dark energy’ proposed as an explanation, but its nature remains a mystery
  • Hubble observations have ruled out one dark energy hypothesis

ASTRONOMERS USING NASA’s Hubble Space Telescope have ruled out one explanation for the nature of dark energy after recalculating the expansion rate of the universe to unprecedented accuracy.

The universe appears to be expanding at an increasing rate. Some think this is because it is filled with a ‘dark energy’ that works in the opposite way to gravity.

An alternative to that hypothesis is that an enormous ‘bubble’ of relatively empty space eight billion light-years wide surrounds our galactic neighbourhood.

If we lived near the centre of this void, observations of galaxies being pushed away from each other at accelerating speeds would be an illusion.

This hypothesis has now been invalidated because astronomers have refined their understanding of the universe’s present expansion rate.

“We are using the new camera on Hubble like a policeman’s radar gun to catch the universe speeding,” said Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University, and leader of the science team. “It looks more like it’s dark energy that’s pressing the [accelerator] pedal.”

Portion of NGC 5584 with Cepheid locations marked

A portion of galaxy NGC 5584 with the location of Cepheid variable stars marked.

The observations helped determine a figure for the universe’s current expansion rate to an uncertainty of just 3.3 percent. The new measurement reduces the error margin by 30 percent over Hubble’s previous best measurement in 2009.

Cosmic yardsticks

Riess’ team first had to determine accurate distances to galaxies near and far from Earth, and then compare those distances with the speed at which the galaxies are apparently receding because of the expansion of space.

They used those two values to calculate the Hubble constant, the number that relates the speed at which a galaxy appears to recede to its distance from the Milky Way.

Because we cannot physically measure the distances to galaxies, astronomers have to find stars or other objects that serve as reliable cosmic yardsticks. These are objects with known intrinsic brightness—brightness that hasn’t been dimmed by distance, an atmosphere or interstellar dust. Their distances, therefore, can be inferred by comparing their intrinsic brightness with their apparent brightness as seen from Earth.

To calculate long distances, Riess’ team chose a special class of exploding star called Type Ia supernovae. These stellar blasts all have similar luminosity and are brilliant enough to be seen far across the universe.

By cross-correlating the apparent brightness of Type Ia supernovae with pulsating Cepheid stars (another class of stars whose intrinsic brightness is known), the team could accurately gauge the distances to Type Ia supernovae in far-flung galaxies.

WFC3

Hubble's latest camera, the Wide Field Camera 3, was instrumental in the research.

Bubble is burst

By using the sharpness of Hubble’s new Wide Field Camera 3 (WFC3) to study more stars in visible and near-infrared light, the team eliminated systematic errors introduced by comparing measurements from different telescopes.

“WFC3 is the best camera ever flown on Hubble for making these measurements, improving the precision of prior measurements in a small fraction of the time it previously took,” said Lucas Macri, a collaborator on the Supernova Ho for the Equation of State (SHOES) Team from Texas A&M in College Station.

Knowing the precise value of the universe’s expansion rate further restricts the range of dark energy’s strength and helps astronomers tighten up their estimates of other cosmic properties, including the universe’s shape and its roster of neutrinos, or ghostly particles, that filled the early cosmos.

“Thomas Edison once said ‘every wrong attempt discarded is a step forward,’ and this principle still governs how scientists approach the mysteries of the cosmos,” said Jon Morse, astrophysics division director at NASA Headquarters in Washington.

“By falsifying the bubble hypothesis of the accelerating expansion, NASA missions like Hubble bring us closer to the ultimate goal of understanding this remarkable property of our universe.”

Adapted from information issued by STScI. NGC 5584 image credit: NASA / ESA / A. Riess (STScI/JHU), L. Macri (Texas A&M University) / Hubble Heritage Team (STScI/AURA). NGC 5584 illustrations credit: NASA / ESA / L. Frattare (STScI) / Z. Levay (STScI).

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Galaxy not so far, far away

Galaxy NGC 247

Galaxy NGC 247 has been found to be 1 million light-years nearer to us than previously thought.

  • Astronomers measure distances to galaxies using Cepheid stars
  • Their brightness is used as a cosmic ‘standard candle’
  • Method brings galaxy NGC 247 one million light-years closer

HOW FAR AWAY is this galaxy? According to astronomers, it is a lot closer than previously thought—in fact, about a million light-years closer.

Distances to nearby galaxies are measured using the properties of a particular kind of star known as a ‘Cepheid variable’.

Cepheids brighten and fade with a regular pattern. Astronomers can plot a Cepheid’s brighten/fade period and then put that into a special formula, which gives them the star’s intrinsic luminosity…that is, how bright it would be if we were up close to it.

By comparing that intrinsic brightness with the star’s actual measured brightness, astronomers can gauge how far away it is.

It’s a bit like being able to judge how far away a car is at night, based on how bright its headlights seem to be.

Astronomers refer to Cepheids as ‘standard candles’, and have used this method for many years to estimate the distances to nearby galaxies. It doesn’t work for very distant galaxies—as we look further out into space, a point is reached where individual stars cannot be made out.

But difficulties remain with the Cepheid method. Recent work has suggested that the Cepheids’ ‘period-luminosity’ relation is not as clear-cut as previously thought.

In addition, a Cepheid’s brightness can be affected by interstellar dust absorbing some of the star’s light, making it seem fainter than it really is.

Close-up of part of NGC 247

Studying Cepheid variable stars in other galaxies enables astronomers to determine the distances to those galaxies.

Dimmed by dust

NGC 247 is a spiral galaxy in the Sculptor Group of galaxies, which is the nearest collection of galaxies to the Milky Way’s own galaxy cluster, the Local Group.

But how far away is it?

One of the good things about NGC 247 is that it is close enough that individual stars can be made out in high-resolution images, making it a prime candidate for Cepheid measurements.

But the galaxy is tilted to our line of sight—we’re seeing it about halfway between face-on and edge-on. This means that its starlight, including that from the Cepheids, has to pass through a lot of the dust inside the galaxy.

An international team of astronomers known as the Araucaria Project is seeking to refine NGC 247’s Cepheid distance measurements by taking the dust into account.

Their initial findings suggest NGC 247 is over 1 million light-years nearer to us than earlier thought. The official distance now stands at a little over 11 million light-years.

The image was produced by combining a number of exposures taken (through different filters) with the Wide Field Imager on the MPG/ESO 2.2-metre telescope at the European Southern Observatory’s (ESO) La Silla Observatory in Chile.

One of the filters brings out clouds of hydrogen gas (coloured pink) along the spiral arms, indicating areas of active star formation.

Written by Jonathan Nally, SpaceInfo.com.au. Image courtesy ESO.

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Pulsating star mystery solved

Artist's impression of OGLE-LMC-CEP0227

Artist's impression of the binary star OGLE-LMC-CEP0227. The smaller of the two stars is a pulsating Cepheid variable and the orientation of the system is such that the stars eclipse each other during their orbits.

A lucky alignment of orbits has enabled astronomers measure the mass of a specific kind of star, and in the process solve a decades-old mystery.

The star in question is of a type known as a ‘Cepheid variable’, or just Cepheid. Cepheids are unstable stars that are larger and much brighter than the Sun. They expand and contract in a regular way, taking anything from a few days to months to complete the cycle.

The time taken to brighten and fade again is longer for stars that are more luminous and shorter for the dimmer ones. This is known as the period-luminosity relation.

Astronomers can use this to their advantage when estimating the distances to galaxies. By timing the rise and fall in light of a Cepheid in a distant galaxy, they can work out how intrinsically bright the star must be.

By comparing that inherent brightness with the actual brightness measured, they can work out how far away the star must be.

The Cepheid period-luminosity relation, discovered by Henrietta Leavitt in 1908, was used by Edwin Hubble to make the first estimates of the distance to what we now know to be galaxies.

More recently Cepheids have been studied with the Hubble Space Telescope and with ground-based telescopes to make highly accurate distance estimates to many nearby galaxies.

The period-luminosity relation makes the study of Cepheids one of the most effective ways to measure the distances to nearby galaxies and from there to map out the scale of the whole Universe.

Large Magellanic Cloud

The Large Magellanic Cloud galaxy, in which the Cepheid star was discovered. Further Cepheid discoveries in this galaxy could help astronomers pin down its distance to better than 1% precision.

Competing theories

Unfortunately, despite their importance, Cepheids are not fully understood. In particular, predictions of their masses derived from the theory of pulsating stars are 20% less than predictions from the theory of the evolution of stars. This embarrassing discrepancy has been known since the 1960s.

To resolve this mystery, astronomers needed to find a binary star containing a Cepheid where the orbit happened to be seen edge-on from Earth. In these cases, known as eclipsing binaries, the brightness of the two stars dims as one passes in front of the other, and again when it passes behind the other star.

In pairs such as these, astronomers can determine the masses of the stars to high accuracy using well-known physical laws.

Unfortunately, neither Cepheids nor eclipsing binaries are common, so the chance of finding such an unusual pair seemed very low. None are known in the Milky Way.

“Very recently we actually found the double star system we had hoped for among the stars of the Large Magellanic Cloud,” says Wolfgang Gieren, a member of the team. The Large Magellanic Cloud is a small galaxy that orbits our Milky Way galaxy at a distance of around 160,000 light-years.

The system “contains a Cepheid variable star pulsating every 3.8 days,” adds Gieren. “The other star is slightly bigger and cooler, and the two stars orbit each other in 310 days.”

The observers carefully measured the brightness variations of this rare object, known as OGLE-LMC-CEP0227, as the two stars orbited and passed in front of one another. They also used spectrographs to measure the motions of the stars towards and away from the Earth—both the orbital motion of both stars and the in-and-out motion of the surface of the Cepheid as it swelled and contracted.

This very complete and detailed data allowed the observers to determine the orbital motion, sizes and masses of the two stars with very high accuracy—far surpassing what had been done before for a Cepheid.

The mass of the Cepheid is now known to about 1% and agrees exactly with predictions from the theory of stellar pulsation. The larger mass predicted by stellar evolution theory was shown to be significantly in error.

The team hopes to find other examples of these remarkably useful pairs of stars to exploit the method further. They think that from such binary systems they will eventually be able to pin down the distance to the Large Magellanic Cloud to 1%, which would mean an extremely important improvement of the cosmic distance scale.

Adapted from information issued by ESO / L. Calçada.

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