August 2021 Volume 47 Number 4 in the Pages of the Gemini
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Minnesota Astronomical Society August 2021 a publication of the Minnesota Astronomical Society Volume 47 Number 4 In the pages of Feint Preys By Dave Tosteson the Gemini Not long ago the elegant structures called globular clusters, those spherical, glittering balls of hundreds of thousands of stars, were all considered remnants What? Two 30-inch scopes? of our galaxy’s formation. Thought to be produced at the same time as the Milky By Dick Jacobson…Page 5 Way twelve-billion years ago, they were like stately, never-changing reminders of a bygone era, antebellum mansions surviving the social and military upheavals of MAS Patron Members…Page 10 a turbulent time. Now we know differently. Not only has our galaxy gained and MAS Board Minutes lost numerous clusters, but we understand globulars to be far from unchanging By Trena Johnson, Secretary…Page 10 remnants. Likely, all have been altered by tidal gravity and interaction with galactic structure. Some estimates place up to forty-percent of our present globulars as We Live in the Stars extragalactic addressees before “adoption” by the Milky Way. Here is a group of By Aaron D. Raines…Page 11 recently recognized Milky Way globulars which can be visually observed, and one I have my eye on. Variable Star of the Month By Jim Fox…Page 12 MAS 2021 Star Party Schedule…Page 12 Loaner Scopes & Astronomy DVDs Available to All MAS Members…Page 14 Fornax dwarf galaxy & globulars In 2002 Alan Whiting and his colleagues published information on their search for faint dwarfs in our galaxy. In the dozens of objects recovered from their Cerro Tololo imaging were galaxies, planetary nebulae and a small collection of faint stars in central Cetus thought to be an open cluster. Three years later, Giovanni Carraro of the University of Padua (Italy) used the VLT to determine Stories Wanted: Gemini is written entirely spectrograms and radial velocities to place this cluster in our galaxy’s halo, thirty by our members, for our members. Gemini needs kiloparsecs from the center. Its metal content profile (of elements heavier than your stories: how you first became interested in helium) and its position and velocity suggested it was a member of a class of astronomy, how your interest has evolved over the six known tidally stripped globulars added to the Milky Way’s retinue from the years, equipment you use, projects or activities captured Sagitarrius Dwarf Spheroidal galaxy (Sag dSph). Before its accretion, you have worked on, star parties or events you’ve its case may have been similar to the Fornax Dwarf galaxy, one with a half-dozen attended in this state and elsewhere, how you’ve globulars that, for amateurs, are much easier to see than the galaxy itself. Fornax’s encouraged others—especially young people—to brightest globular, NGC 1049, was found a century before the galaxy was spotted get involved in this fascinating hobby. Submit your by Shapley in 1938; it could be considered a correlate of M54, the former core of stories to [email protected] the Sag dSph galaxy. It and Fornax represent dwarf galaxies that have succumbed If your e-mail, phone number or street to and resisted the Milky Way’s take-out menu. The idea of interacting dwarfs address has recently changed, please fill out contributing to our census of globular clusters dates to Searle and Zinn in 1978. the “Update Member Information” Web form Whiting-1 was named in honor of its discoverer, Alan B. Whiting of the at mnastro.org/update-member-information to Cerro Tololo Interamerican Observatory. This small (one arcminute/three update your contact information in the MAS parsecs) cluster has a loose structure. Its brightest star is magnitude (v) 18.0 member records. and its horizontal branch, where resolution would start to GEMINI INFO occur in the eyepiece, is 20.4. Editors At 6.5 billion years old it is the Brian Litecky and Eugene Brown youngest Sag dSph globular and one of the youngest in the Webmaster/MAS Web Committee: Milky Way. At the Okie-Tex [email protected] Star Party in September, 2006, Forums Administrator I used my newly acquired 32'' Russ Durkee reflector to spot several of the Whiting-1 stars in a near-dawn Monthly Meeting Presenter observation with the Moon up Coordinator and the object at an altitude of Ahmed Reda only thirty degrees. Its position of 02h 02m 57s, -03d 15m 08s Astronomical League Coordinator (ALCOR) sits on the SE edge of Abell Jerry Jones Galaxy Cluster 296 and 4.1 degrees west of Omicron Ceti, Outreach Coordinator Mira. Catching the cluster at Lilah Blinkman its culmination would provide a view fifty degrees above the Gemini is published 6 times annually Koposov-1 horizon from that latitude, by the Minnesota Astronomical Society. offering improved chances for Electronic submissions for seeing its fainter stars. Gemini may be sent to: In the first years of the new Millennium, searches by Whiting and others using [email protected] the Sloan Digital Sky Survey (SDSS) produced a spate of objects that orbited the Hardcopy items should be sent to: Milky Way. Simulations to help explain the “missing mass” problem had predicted Minnesota Astronomical Society thousands of such galaxies and clusters, but researchers were finding only a few- Attn: Gemini dozen dwarf galaxies. Reevaluation with computer programs that added gas and stellar P.O. Box 14931 components to the previous dark-matter-only scenarios reduced the number by an order Minneapolis, MN 55414 of magnitude. Fewer than a dozen in the “FIRE” simulation were predicted to form stars within the dark-matter component. Interaction with our galaxy’s halo appears to have interfered with stellar production in some of these proto-clusters, leaving only MAS Board Members starless, dark-matter clumps. Searches in the halo of the Andromeda Galaxy, which in some ways are easier to perform than in the Milky Way, found globulars to distances President: Mark Job of 650,000 light years from its center, exemplified by Martin GC1. Some predict that E-Mail: [email protected] intermingling of halo globulars between the Milky Way and our sister galaxy will be Vice President: Valts Treibergs discovered. E-Mail: [email protected] Factors differentiating the new finds into either globular clusters or dwarf galaxies include mass, luminosity, size, mass/light ratio, ellipticity and the spread of metal Treasurer: Matt Dunham content within their stars. A key difference is that dwarf galaxies seem to possess large E-Mail: [email protected] amounts of dark matter and globulars do not. This unseen mass allows them to withstand Secretary: Trena Johnson the disruptive forces of gravitational tides and supernovae and increases the spread E-Mail: [email protected] of metallicity by incorporating heavier elements made by subsequent generations of stars. Most clusters are thought to have formed concurrently with their parent galaxies, Board Member at Large: and theoretical work at the University of California, Riverside, suggests 50-60% of Gunnar Isberg these early globulars formed within their own dark-matter halos, which were later E-Mail: [email protected]. removed through processes such as dynamical friction, mass segregation and tidal Board Member at Large: stripping. Waves of star formation within galaxies and accretion of external objects Conrad Sanders may be represented by the differing ages of any particular galaxy’s cluster population. E-Mail: [email protected] Most Milky Way globulars have stars with ages commensurate with that of their galaxy. But there are outliers within that group, both in characteristics and position. These objects that don’t fit the usual profile are of great interest to researchers, as they hold clues to their origin and evolution. Palomar-15 shows strong evidence of tidal stripping, with a stream of stars trailing behind it. Messier’s same number shows signs of internal reshaping. The Sloan Digital Sky Survey (SDSS) imaged one-quarter of the sky from its New Mexico site and has been a discovery source for many faint objects in and around our galaxy. In 2007 Sergey Koposov (then at Max Planck Institute in Heidelberg, Germany, later at Cambridge University, UK) and his colleagues announced the discovery of two faint, halo globular clusters (ApJ, 669, 337) at galactocentric distances of 40-50 kpc, subsequently named Koposov-1 and Koposov-2. At the time, GEMINI • www.mnastro.org 2 Koposov-2 beat out AM-4 for the title of lowest luminosity for stellar population of twelve-billion-year-old stars and contained a Milky Way globular, with an absolute magnitude (Mv) of no dark matter. Stellar members found at large distances from -0.35 (compared to -1.4 for AM 4). Koposov-1 came in third at its center suggested ongoing mass loss. The SEGUE project’s -2.0, later adjusted to -1.35. For comparison, Omega Centauri, first named find was an Ultrafaint Galaxy called SEGUE-1 that our galaxy’s largest and brightest globular, has an Mv of -10.3, contained only a few hundred stars, with the highest mass-to- several thousand times brighter. Of great interest about these light ratio of any known object at the time—over three-thousand. newly discovered globulars is their short “evaporation time,” the It was basically composed of only dark matter, but I was able period expected for them to lose all their stars into the halo and to spot a few of its identified stellar members in 2009 using my intragalactic medium. This estimate for the Koposov clusters is 32'' scope. There are likely numerous dark-matter-only objects a little over a billion years, which suggests misfortune in both orbiting our galaxy and within the Local Group, but since they their affairs and that of the primordial globular population of our would produce no electromagnetic radiation, they will be very galaxy—a flood of clusters lost through tides since the Milky hard to detect, offering amateurs nothing to see.