(This note written 10 years ago, like to share it here.)
M13 Globular Cluster at visual and IR wavelengths
Location: Hercules RA 16h 41.7m Dec +36° 28'.
Distance 25,000 light years Visual Magnitude 5.8
Apparent diameter ~20 arcmin True diameter ~140 light years
In the near-infrared image by 2MASS* (wavelength 1 – 2 microns), we see many more stars than the visible-light image, because globular clusters in the Galaxy comprise primarily of old stars in which the peak emission is near-infrared.
In the mid-infrared (12 microns), the relatively poor spatial resolution of the IRAS** detectors results in the smearing of the light from M13 into a central blob. Note that this emission is not spherically symmetric like the underlying star cluster itself. This is an artifact resulting from the rectangular shape of the IRAS detectors. (If these detectors were square instead, the image would be circularly symmetric.)
In the far-infrared image taken at 60 microns with IRAS, M13 has vanished ! This is because globular clusters generally have no young stars and very little dust. Young stars and dust do not emit infrared longer than about 30 microns.
----------------
* 2MASS = The Two Micron All-sky Survey. It last from 1997 to 2001 at two 1.3 m telescopes located one each in the northern and southern hemispheres.
** IRAS = Infrared Astronomical Satellite, a joint US-Dutch-British satellite launched in 1983, which made a sky survey at wavelengths of 12, 25, 60 and 100 microns. It carried a 57-cm RC telescope and an array of detectors cooled to ~10 K by liquid nitrogen. It served for 10 months.
發表於 2010-9-7 14:52:20
)，它在我們鄰居 LMC (大麥哲倫雲) 內，所以較易觀測，研究者說 NGC 1818 的年齡低至 0.2 ~ 0.4 億年，比傳統的球狀星團年輕幾百倍 ! 過往我們一直知道 LMC 有年老及年輕的星團共存，NGC 1818 只是年輕中較出眾的，HST 有了它的照片後便引為傳媒話柄。依此思路，LMC 應是 "頻密演化" 中的年輕星系， "頻密演化" 由星系併合蓄成，難怪一些天文學家懷疑銀河系也是星系併合而來的，不過這些都是懷疑，還末有肯定。