Messier 45 is more commonly referred to as the Pleiades, and is the most famous open star cluster among amateur astronomers and the general public alike. Easily visible to the naked eye during the northern Winter, this cluster has been featured in literature over three millennia! Research compilations now suggest that about 500 stars, mostly faint, are gravitationally bound together as the Pleiades.
Visible: DSS, Visible: Color - © AAO, Royal Observatory, Edinburgh
Let us begin our examination of Messier 45 with a look at the visible light images (above). The black-and-white and color images both reveal wispy veils of fog surrounding the brightest stars. These reflection nebulae are produced by dust particles within the cluster. The chemical composition of the reflected light (as determined through spectroscopy) is identical to that of the blue stars, thereby confirming that the nebulae are simply stellar light reflected to our line of sight by the dust. The blue tint is a consequence of the fact that the illuminating stars are young and blue, and because dust preferentially reflects light of shorter wavelengths (for example, blue) and preferentially scatters light of longer wavelengths (such as red). The spikes seen around the brightest stars, particularly in the color image, are artifacts produced by the telescope optics.
Near-Infrared: 2MASS and Visible: DSS
Now ponder the near-infrared photograph (above left) and compare it with the previously studied visible-light picture (above right). The pattern of the five brightest stars is still clearly seen. However, the surrounding haze has completely vanished! This is because near-infrared light, corresponding to wavelengths a few times longer than what the human eye sees, can easily pierce through the obscuring effects of dust. This is an important reason why astronomers often rely on near-IR radiation to study the birth of stars, since stellar formation normally occurs within thick cocoons of dust and gas.
Mid-Infrared: IRAS and Far-Infrared: IRAS
The IRAS infrared images depicted above were obtained at wavelengths of 25 and 60 microns. Two dramatic differences from the previous images are immediately obvious. First, the five-star pattern is barely recognizable in the mid-infrared (upper left) and has disappeared at far-infrared wavelength (upper right). Second, much of the infrared emission now corresponds to the dust wisps noted in the visible light photos. This is the paradox of infrared light. At the shortest wavelengths, near-IR light effectively passes through obscuring dust. At the longer wavelengths, infrared emission is increasingly due to the dust particles themselves. At these wavelengths, the dust particles absorb the ambient visible and ultraviolet photons emitted by the nearby stars. The dust then re-radiates the light as infrared, with the energy difference between the types of light serving to slightly heat the dust particle. [In both of these infrared images, a point of light is stretched along the east-west direction because of the peculiar rectangular shape of the IRAS detectors. Furthermore, the pattern of stripes is a result of data processing.]
Mid-Infrared: IRAS and Visible: Color - © AAO,
Royal Observatory, Edinburgh
If you need additional evidence that longer-wavelength infrared originates primarily from dust, re-examine the mid-IR and visible-light images above. Concentrate on the southernmost bright star in the Pleiades, named Merope. You will see that the fuzzy pattern of infrared light to the immediate southwest (lower right) of this star closely resembles the pattern of reflected light seen in the visible light photograph.
Radio: NVSS, Ultraviolet: MSX, X-Ray: Thomas Preibisch, Wurzburg (Germany)
The radio image (above left) of Messier 45 shows some faint point sources of emission, color coded as green. [The black and blue background is simply noise, analogous to radio static.] The pattern of the point sources does not clearly match the brightest stars seen in the visible and near-infrared photos. What are these sources? Good question, with no obvious answer. Many distant quasars are strong radio emitters. However, a search of the online NASA/IPAC Extragalactic Database (NED) fails to uncover any cataloged background quasars within the field of view. Some of the faint radio emissions could originate from distant and uncataloged quasars. Most quiescent stars are not known for being strong sources of radio emission. However, such unusual stars as flare stars, pulsars, and binary stars can produce radio signals, and these could account for some of the sources seen in the Messier 45 image.
In the ultraviolet (above center) we see the emission from hot stars. The dust around newly formed stars reflects and scatters ultraviolet light. Ultraviolet images can provide information about the properties of dust surrounding newly formed stars.
Finally, let us study the x-ray image (above right) of the Pleiades. The square green boxes
denote the position of the brightest stars in the cluster. Some of the boxes
are contain a faint source, suggesting that the x-rays are due to the
(optically) bright star. However, other boxes appear to contain no x-ray
emission. In fact, most of the x-ray sources (color coded to denote different
x-ray energies, or wavelengths) are widely scattered throughout the field of
view, although clearly centered about the cluster position. Research
astronomers have found that the majority of x-ray sources are faint, low-mass
stars within the Pleiades.