Cepheid variables are simply indispensable. It was Harvard’s Henrietta Leavitt who, in 1912, discovered a relationship between the cycle of variable brightness in these stars and their luminosity. With a classic Cepheid, the longer the period of the star, the greater its intrinsic brightness. That sets up the method: Determine the period of the variable, check its apparent magnitude with the absolute magnitude corresponding to that period, and you can measure the distance. The relevant term is ‘standard candle.’
But put telescopes into space and you can refine these measurements, as studies of Cepheid variables with the Hubble Space Telescope have now shown. That’s helpful because we’d like to know the Hubble constant — the universe’s rate of expansion — as accurately as possible, and Cepheids are one of our best tools. To fine-tune the Cepheid method, a team from the University of Texas at Austin has directly measured the distance to ten Cepheid variables, using Hubble to trace their apparent motion in the sky, called parallax.
Parallax has a distinguished history. Looking at a star from opposite sides of Earth’s orbit around the Sun, astronomers made early distance measurements by seeing how far the star was displaced — the star seems to make a small circle on the sky. That apparent motion is helpful for calculating the distance to nearby stars, but far trickier when measuring more distant ones. The Texas’ team’s Milky Way targets were tough — the circle they drew was the equivalent of a quarter seen at 1500 miles — demanding the use of Hubble’s Fine Guidance Sensors.
But once achieved, the parallax findings give us a precise distance that can be weighed against the intrinsic brightness measurements for the Cepheid, and that helps us tune the period-luminosity relationship. Having made the calibration, astronomers can more accurately deduce the distance to Cepheids in distant galaxies. Ultimately, using such data should improve the accuracy of the Hubble constant, giving us precise measurements to any galaxy whose redshift can be measured.
There are other standard candles besides Cepheid variables, and I want to look at some of these in a future post. For now, though, the paper is Benedict et al., “Hubble Space Telescope Fine Guidance Sensor Parallaxes of Galactic Cepheid Variable Stars: Period-Luminosity Relations,” Astronomical Journal 1816 (April 2007), pp. 1810-1827. Abstract available.
Way back in the 50s astronomers found an error in the Cephied period-luminosity relationship and the Andromeda galaxy got moved from 1 million to a bit over 2 million LY away. Glad to see ongoing refinements and a DIREST calibration mesurement of 10 Cephieds.
Yes, amazing how the scale of the universe changed again and again during the 20th Century as we worked out these measurements!
Hubble is an impressive instrument, though astrometry is not it’s main mission it does produce fantastic results. The biggest gains in this area recently have actually come from the Hipparcos mission, a dedicated astrometry mission which produced a superb catalog of stellar distances and other measurements. The successor to Hipparcos, GAIA slated for a 2011 mission launch will represent the first real attempt to begin cataloging 3D positions of all of the stars in our galaxy (GAIA will catalog about 1% of those stars). Complementing GAIA’s broad, billion star coverage, the Space Interferometry Mission (SIM) will be able to measure distances to stars on the other side of the Milky Way to within 10% accuracy, but will only be able to sample a much smaller selection of stars (due to brightness / throughput constraints).
Much like our exoplanet catalog, our astrometric catalog has been expanding at a geometric rate in recent years and will continue to do so for the foreseeable future. The explosion of knowledge we will have of our local stellar neighborhood and of our Galaxy and, indeed, even our galactic neighborhood in the next 10, 20, and 30 years is almost too much to believe. The age we live in…
A fine and helpful message indeed, Robin. Both SIM and GAIA should be major stepping stones as we refine our measurements. And on a longer time-frame, one of the advantages of pushing outside the solar system will be the ability to set up truly long baseline interferometry, again extending our understanding of actual distances in the Milky Way and beyond. Fascinating and long-term work.
We will see huge advantages even before we extend our industrial base outside our own Solar System. Parallax is linear with respect to both distance (inversely) and baseline. Today, in order to measure the parallax of stars on the other side of the galaxy requires precise astrometric measurements with an accuracy of several micro-arc seconds (mas), ~2mas for +/- 10% accuracy. Today such fine measurements require exquisite instrumentation and advanced technologies. But simply expand the baseline (today 1AU) by a factor of 10, 100, or 1000 and the requirements on your astrometric instruments drop down dramatically (and their accuracy at shorter distances increases all that much more).
A baseline of a single light year represents a factor of improvement of nearly 4 million in this equation. To put that in perspective, performing all-sky surveys using very cheap, relatively modest telescopes in high-throughput systems (such as SDSS uses, with a resolution of ~0.5 arc-seconds) you would be able to match the precision in distance which with a single AU baseline requires advanced technologies such as used by SIM. Now imagine, if you dare, something like a successor to GAIA with sub 20-microarcsecond resolution in a fleet, flying out at different direction from Earth at speeds greater than that of today’s Pluto Express probe. Creating full sky surveys from multiple observation points with average parallax baselines growing at several AU per year. It’s almost impossible not to imagine that such a mission, or it’s near equivalent, will not be undertaken within the next 20 or 30 years.
One of the many benefits of much longer baseline astrometry surveys will be a deeper understanding not just of the 3D positioning of stars throughout our galaxy but also of the 3D positioning of non-stellar structures (dust, gas, nebulae, etc.). Such data cannot help but radically transform our understanding of the processes which make up, essentially, the metabolism of our galaxy (cycles of stellar birth, life, and death, patterns of interactions of stars, gases, and dust, distribution of dark matter within our own galaxy, etc.). We will discover galactic processes and phenomena which today we do not even have the vocabulary to describe. Meanwhile, there is our ever growing capacity to catalog other stellar systems (from today’s crude measurements of the presence and orbits of the most massive planetary bodies in the system, to tomorrow’s spectral data and imaging of the stellar system (which will reveal not only planets but also transient phenomena such as comets and impact events), to the future’s imaging of the planetary bodies themselves. And then, finally, to sending instruments and people to other stellar systems directly. Pairing these two things together it’s almost hard to believe how quickly our understanding of the universe outside our own planet and our own Solar System has grown and will grow.
Not too long ago the idea that the night sky and the stars was unknown and unknowable was essentially self-evident. Today we are grasping at that understanding, tomorrow there will be maps, guides, and textbooks which detail the structure, organization, and functioning of our galaxy down to a level of planetary systems. This is the stuff that science fiction is made of, and yet we can see with a few simple equations that we cannot help but to have such data (and understanding, and experiences) in the next few generations.
And, as our knowledge of our galaxy and especially of our nearby stellar neighborhood increases to the point where we know every single planet and moon within 10, 20, 30 light years in exquisite, intimate detail it is hard to imagine a future human civilization which does not say to itself “let’s go!”
Hipparcos had a tiny mirror. It was limited to bright, nearby objects. That is if anything hundreds of light years away can be considered nearby.
HST’s 2.4 meter mirror is so much bigger. So it can get position information that is so much better. But, it isn’t designed to survey the sky for astrometry, it can do basically one star at a time. And so only a few important stars have been measured.
So, you can do better with a bigger mirror (or interferometry). You can do better with a bigger baseline (send a pair of scopes out to Jupiter’s orbit, for example).
If the SETI search finds something, then maybe ET could send us dated pictures of distant galaxies. We then compare them with our own, and get distances based on light years of baseline. I’ve heard it said that ET won’t have much to tell us scientifically. They have lots to tell us.
The Cepheid Galactic Internet
Authors: John G. Learned, R-P. Kudritzki, Sandip Pakvasa, A. Zee
(Submitted on 2 Sep 2008 (v1), last revised 2 Sep 2008 (this version, v2))
Abstract: We propose that a sufficiently advanced civilization may employ Cepheid variable stars as beacons to transmit all-call information throughout the galaxy and beyond. One can construct many scenarios wherein it would be desirable for such a civilization of star ticklers to transmit data to anyone else within viewing range.
The beauty of employing Cepheids is that these stars can be seen from afar(we monitor them out through the Virgo cluster), and any developing technological society would seem to be likely to closely observe them as distance markers. Records exist of Cepheids for well over one hundred years.
We propose that these (and other regularly variable types of stars) be searched for signs of phase modulation (in the regime of short pulse duration) and patterns, which could be indicative of intentional signaling.
Comments: 5 pages, 3 figures
Subjects: Astrophysics (astro-ph); High Energy Physics – Phenomenology (hep-ph); Popular Physics (physics.pop-ph)
Cite as: arXiv:0809.0339v2 [astro-ph]
Submission history
From: Sandip Pakvasa [view email]
[v1] Tue, 2 Sep 2008 09:26:39 GMT (30kb)
[v2] Tue, 2 Sep 2008 23:16:12 GMT (30kb)
http://arxiv.org/abs/0809.0339