Separating Observer from Observed

One of the most difficult things to do in observational science is to separate the observer from the observed.  For example, in CCD astronomy, we apply bias, dark, and flat-field corrections as well as utilize median combines of shifted images to yield an image that is, ideally, free of any CCD chip defects including differences in pixel sensitivity and zero-point.

We as observers are constrained by other limitations.  For example, when we look at a particular galaxy, we observe it from a single vantage point in space and time, a vantage point we cannot change due to our great distance from the object and our existence within an exceedingly short interval of time.

Yet another limitation is a phenomenon that astronomers often call “observational selection”.  Put simply, we are most likely to see what is easiest to see.  For example, many of the exoplanets we have discovered thus far are “hot Jupiters”.  Is this because massive planets that orbit very close to a star are common?  Not necessarily.  The radial velocity technique we use to detect many exoplanets is biased towards finding massive planets with short-period orbits because such planets cause the biggest radial velocity fluctuations in their parent star over the shortest period of time.  Planets like the Earth with its relatively small mass and long orbital period (1 year) are much more difficult to detect using the radial velocity technique.  The same holds true for the transit method.  Planets orbiting close to a star will transit more often—and are more likely to transit—than comparable planets further out.  Larger planets will exhibit a larger Δm than smaller planets, regardless of their location.  It may be that Earthlike planets are much more prevalent than hot Jupiters, but we can’t really conclude that looking at the data collected so far (though Kepler has helped recently to make a stronger case for abundant terrestrial planets).

Here’s another important observational selection effect to consider in astronomy: the farther away a celestial object is the brighter that object must be for us to even see it.  In other words, many far-away objects cannot be observed because they are too dim.  This means that when we look at a given volume of space, intrinsically bright objects are over-represented.  The average luminosity of objects seems to increase with increasing distance.  This is called the Malmquist bias, named after the Swedish astronomer Gunnar Malmquist (1893-1982).

Intergalactic Stars

Did you know that a few percent of all stars are traveling alone through intergalactic space, no longer a part of any galaxy?  Gravitational interactions between stars or between stars and black holes can occasionally accelerate a star to galactic escape velocity so that it is thrown (eventually) into intergalactic space.  When the star first enters intergalactic space, the view of your home galaxy would be pretty remarkable, but eventually (eons later, of course) there would be very few naked eye objects in your night sky. Just moons and planets, meteors, aurora, comets, the zodiacal light, and maybe a galaxy or two. Anything else would require a telescope.  And an observer, of course.

The first evidence for intergalactic stars came from the detection of diffuse light between galaxies (Zwicky 1952).  Much later, intergalactic planetary nebulae were detected in the Fornax galaxy cluster (Theuns & Warren 1997).  More recently, intergalactic red giant stars have been detected in the Virgo galaxy cluster using the Hubble Space Telescope (Ferguson et al. 1998).

The Fornax cluster lies about 62 million light years distant, and the Virgo cluster 54 million light years distant.  Have any intergalactic stars been detected near our Milky Way galaxy?  Brown et al. (2005) discovered the first hypervelocity star, SDSS J090745.0+024507, a 20th-magnitude star in the constellation Hydra.  Though it is just 160,000 light years from the center of our galaxy, it is moving away from the Galactic center at an astonishing radial velocity of 709 km/s.  Even though this one-dimensional radial velocity1 is only a lower limit to the star’s true 3D space motion, it is far and away fast enough to escape our Milky Way galaxy altogether.  Gaia will probably be able to measure this runaway star’s proper motion in right ascension and declination, thus allowing a determination of the true space velocity of SDSS J090745.0+024507 relative to the Galactic center.

Several more hypervelocity stars have been discovered since 2005.  One of them, US 708, a 19th-magnitude white dwarf in Ursa Major, is exiting our galaxy at a velocity of at least 1200 km/s!  This makes it the fastest on record (Geier et al. 2015).

1The observed one-dimensional radial velocity as seen from Earth is corrected for the Earth’s rotation and motion around the Sun, and the Sun’s motion around the center of the Milky Way galaxy to determine the galactocentric radial velocity.

Brown, W. R., Geller, M. J., Kenyon, S. J., Kurtz, M. J. 2005, ApJ, 622, L33
Ferguson, H. C., Tanvir, N. R., & von Hippel, T. 1998, Nature, 391, 461
Geier, S., Fürst, F., Ziegerer, E., et al. 2015a, Science, 347, 1126
Theuns T., Warren S. J., 1997, MNRAS, 284, 11
Zwicky F., 1952, PASP, 64, 242