1 / 13

The Center of the Milky Way

The Center of the Milky Way. Louis Gao Boston Latin School 5.12.07 Chandra Astrophysics Institute. Introduction. Milky Way is “home galaxy” Naturally a topic of interest among astronomers X-ray, radio telescopes reveal extremely energetic object behind dust clouds obscuring galactic core

fleur
Download Presentation

The Center of the Milky Way

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. The Center of the Milky Way Louis Gao Boston Latin School 5.12.07 Chandra Astrophysics Institute

  2. Introduction • Milky Way is “home galaxy” • Naturally a topic of interest among astronomers • X-ray, radio telescopes reveal extremely energetic object behind dust clouds obscuring galactic core • Something fearfully powerful at the center of the galaxy? • (How big is it, anyway?) • Black Holes? • Intrinsically mysterious • Indiscriminately destructive? • (What if one was in Milky Way?)

  3. The Anatomy of the Black Hole • The “black hole” is region of space from which nothing returns • (black region at right) • The “accretion disk” is layer of infalling material • friction causes light emission (multi-colored region at right) • Event horizon • “point of no return”

  4. Methods • Galactic center inaccessible by visible telescopes • CXO (X-ray) images • SAOImage DS9 to analyze CXO images • Light Curve Analysis Tool • Graph of “Photon Density” vs. Time • BH Mass • Mass, radius of “black” portion of black hole precisely, directly correlated • 3 km additional radius per solar mass • Angular Size • Sun very large, moon very small, same size in night sky? • Angular (apparent) size based on distance, actual size of the object • Light Travel Time • Light has finite speed– how long for light to travel across an object?

  5. Theorycraft

  6. Results (Mass Radius) • Mass SGR A*: 3.7 million solar masses (Ghez et al., 2004) • If 3 km radius per solar mass, then Radius: 11.1 x 106 km • 11.1 x 106 km = 1.11 x 1010 m!

  7. Results (Angular Size) • Two factors combine for Angular Size • Actual size (If it IS smaller, looks smaller) • Distance (If it’s far away, looks smaller) • Radius of circle = 2.338 computer pixels • Each pixel in CXO image = 0.492 arcseconds • Distance to Galactic Core: 8 parsec (Reid 1993) • Can use this information to reverse-solve for distance! • Final calculation for radius: 1.37 x 1015 m! Left: Distance (X) and actual size (Y) combine for an apparent size. An increase in X OR a decrease in Y reduces the apparent size of the object. Left: SGR A* is located within the green circle, the properties of which are shown in the window.

  8. Results (Light Travel Time) • Sgr A* (center of the galaxy) • Light Curve indicates “flares” approx. every 50,000-60,000 sec. (14-16 hours) • “Flares” themselves approximately 6,000 seconds in width • Big flare also 6,000 seconds in width • If flare instant, then 6,000 seconds for light to go across SGR A*-- must have diameter of 6,000 light seconds! (radius 3,000 ls!) • 3,000 ls = 9 x 1011 meters! Left: SAOImage DS9 Light Curve of SGR A*, with magnified flare circled in RED. Right: Magnified flare from left, showing clearly the duration of about 6,000 seconds.

  9. Analysis/Discussion • Mass Radius estimate: 1.11 x 1010 meters • Angular Size estimate: 1.37 x 1015 meters • Light Travel Time estimate: 9.0 x 1011 meters • Comparison • Earth Orbit: 1.5 x 1011 meters • SGR A* is very large… • Distance to Alpha Centauri (nearest star to Sun): 3.6 x 1016 meters • Any of estimates feasible via this metric • However… • Periapsis of closest star to SGR A*: 2.24 x 1012 meters (Ghez et al. 2004) • Angular Size estimate implies passage of star THROUGH SGR A*, effectively impossible (therefore invalid) • LTT, MR estimates still fit this observation

  10. Analysis / Discussion (cont’d) • LTT estimate ignores inability for direct passage of light through BH core • Cannot compensate for this, light mechanics around edge of BH not completely known • Therefore, LTT estimate inherently unreliable, imprecise (but good, for the most part, if only 1 order of magnitude off from “accurate” SR estimate) • MR estimate bounds precise radius of event horizon • Event horizon not what is seen when observing imagery of SGR A* • However, estimate bounds meaningful portion of BH

  11. Wait! - • These three methods do not all measure the same thing! • Mass Radius only measures BLACK portion of black hole (accretion disk not relevant to MR) • Light Travel Time only measures VISIBLE part of black hole (cannot use LTT on invisible part) • Angular size only measures PHOTOGRAPHED part of black hole (can only measure what is in the image) • The three figures for size will all be different.

  12. Conclusions • Radius of SGR A* somewhere between 1010 – 1011 m • Further precision almost impossible • Angular size estimates completely inaccurate w/ existing images due to imaging issues • Even very small (point) sources of light look large in CXO • LTT argument imprecise with BH due to light mechanics • Only realistic avenue for further refinement: mass determination • Gravitational effects on nearby objects

  13. Acknowledgements • Thanks to: • NASA • Chandra X-ray Observatory • CAI Staff • For … everything! Yeah! • Viewers Like You • Thank You!

More Related