The cosmic distance scale needs adjustment.
First, some background.
The basic yardstick to figure the distance to other galaxies is a type of pulsating star called a Cepheid variable. A Cepheid’s brightness changes on a regular schedule. Depending on the star, the period can be between one day and a few months, during which its brightness can double from its dimmest output, then drop off again.
The type is named for the closest member of the group, Delta Cephei. Delta, the fourth letter in the Greek alphabet, means it is the fourth-brightest star in the constellation Cephus the king. Through parallax, a method of triangulation that works with relatively nearby stars, the distance to Delta Cephei has been established at about 890 light-years. The star is a yellow supergiant, and, according to the American Association of Variable Star Observers, throbs from its dimmest to brightest magnitude and back again in 5.366 days.
The luminosity change results from the star’s expanding and contracting.
Let’s veer off track a little and pay tribute to the discoverer of Delta Cephei’s pulsations. He was John Goodricke, in infancy the victim of an illness that left him deaf and mute. Unlike most such handicapped children born in 1760s England, Goodricke was lucky, AAVSO says: his wealthy parents made sure the boy got a good education. He proved to be a brilliant student.
At age 17 he began assisting a neighbor astronomer, his friend and mentor Edward Piggot, in searching for variable stars. A paper the young man wrote on the subject was read before the Royal Society of London.
“Defying the odds of prejudice[d] belief, Goodricke was honored by the Royal Society of London in 1783 when he was presented the prestigious Godfrey Copley science medal for his work in determining the period and cause of variation in the great variable star of Algol,” the association adds.
According to the on-line Complete Dictionary of Scientific Biography, Goodricke suggested that Algol is a pair of stars locked in orbit about each other in a configuration that allows one to pass between the other and our vantage on Earth. The periodic eclipsing causes Algol’s brightness to change. More than 100 years later, Goodricke’s theory was vindicated by spectrometry, which recorded the different spectrographs of each of the stars in the Algol system. Pairs of this type are now known as eclipsing binaries.
More important for cosmology, Goodricke discovered that Delta Cephei also is a variable. The Cepheids vary in brightness not because of eclipses but because of pulsations that make stars of this type grow brighter and dimmer as they swell and shrink.
A chart of the variations looks like a shark’s fin, with a gradual increase in brightness and then a sharp falling off. The shape helps distinguish Cepheids from other kinds of variable stars.
“Sadly,” AAVSO notes, “Goodricke died at the tender age of 21 after a bout with pneumonia which he reportedly caught while observing delta Cep in 1786.”
A Harvard astronomer, Henrietta Leavitt, discovered more than 2,400 variables. In 1912, having studied photos of myriads of Cepheids, she determined that the speed of the star’s heartbeat depends on its brightness. The brighter one is, the slower its pulse.
The discovery gave scientists a so-called “standard candle” to measure the distance to stars that are too far away to triangulate. Simply time a Cepheid’s period and you can use Leavitt’s formula to figure out how bright it is. Calculate how much the brightness is reduced by distance and you know how far away it is.
In the 1920s Edwin Hubble used Cepheid variables to discover the nature of galaxies, one of the huge leaps forward in understanding the universe. NASA says in a short biography of Hubble, posted on the web:
“Most astronomers of Hubble’s day thought that all of the universe – the planets, the stars seen with the naked eye and with powerful telescopes, and fuzzy objects called nebulae – was contained within the Milky Way galaxy. Our galaxy, it was thought, was synonymous with the universe.
“In 1923 Hubble trained the Hooker telescope on a hazy patch of sky called the Andromeda Nebula. He found that it contained stars just like the ones in our galaxy, only dimmer. One star he saw was a Cepheid variable, a type of star with a known, varying brightness that can be used to measure distances. From this Hubble deduced that the Andromeda Nebula was not a nearby star cluster but rather an entire other galaxy, now called the Andromeda galaxy.”
From that realization sprang our understanding that the universe hosts many billions of galaxies.
Patrick Wiggins, secretary-treasurer of the Salt Lake Astronomical Society and a NASA solar system ambassador to Utah, recently photographed the star in the Andromeda Galaxy that Hubble realized was a Cepheid in 1923. He shot it in November 2010 when it easy to see on his photo, and again the next month when it was dimmer.
“I was inspired to shoot the images after seeing BYU’s animation of the variable which was screened at a SLAS meeting last year,” he noted. Brigham Young University astronomer Michael A. Joner showed the “movie” of the variable on Nov.16, 2010.
[Patrick Wiggins made this view, cropped here, of Hubble’s variable star in the Andromeda galaxy, indicated by an arrow, in November 2010, when it was brighter. The galaxy’s center is to the left, beyond the edge of the frame]
[He took this view of the variable, also cropped, in December 2010, when it was dimmer. In both images, small dark marks are minor camera defects]
The variable is hard to see even at its brighter stage, because it’s a single star in a galaxy at an estimated 2.3 million light-years away.
But is 2.3 million light-years the real distance?
On Wednesday NASA’s Jet Propulsion Laboratory, Pasadena, announced that the Spitzer Space Telescope had made photographs showing that Cepheids shrink slightly in mass over time. “The telescope’s infrared observations of one particular Cepheid provide the first direct evidence that these stars can lose mass – or essentially shrink,” says a JPL press release written by Whitney Clavin.
The particular star was none other than the granddaddy of all Cepheids, Delta Cephei.
Scientists believe, in the words of the release, that “winds from a Cepheid star could blow off significant amounts of gas and dust, forming a dusty cocoon around the star that would affect how bright it appears. …
“Follow-up observations of other Cepheids conducted by the same team using Spitzer have shown that other Cepheids, up to 25 percent [of those] observed, are also losing mass.”
If Delta Cephei’s brightness is different than was thought — because its gleam is partly obscured by the dusty cocoon and because it is shrinking as material is blown off it — the luminosity of this star 890 light-years away is not what was believed. If the brightness ratio isn’t right, present estimates of distances to other galaxies are wrong.
“Everything crumbles in cosmology studies if you don’t start up with the most precise measurements of Cepheids possible,” NASA quotes Pauline Barmby of the University of Western Ontario, lead author of the follow-up Cepheid study. “These studies will allow us to better understand these stars, and use them as ever more precise distance indicators.”