To get you thinking...

Transcription

1 T get yu thinking... 1.) What is an element? Give at least 4 examples f elements. 2.) What is the atmic number f hydrgen? What des a neutral hydrgen atm cnsist f? Describe its "mtin". 3.) Hw des an atm becme "excited"? When it becmes "un-excited", what des it d? 4.) What is spectrscpy? Hw is it used n Earth? Hw can it be used by an astrnmer?

2 Neutral Hydrgen - As yu (shuld) knw, neutral hydrgen cnsists f a single prtn and a single electrn rbiting the prtn This electrn is fund rbiting in the 1S rbital - There is a secnd mtin besides the rbit f the electrn arund the prtn This is true fr all atms, but we will fcus n Hydrgen as it is much simpler t explain

3 "2nd" Mtin: - Bth the prtn and the electrn are ging t have an individual spin The spin f bth can therefre be in the same directin (aligned) r in ppsite directins (anti-aligned) Because f quantum mechanics, it turns ut when the spins are aligned, the hydrgen is higher in energy

4 Aligned vs. Anti-Aligned - Even thugh the Aligned versin is higher in energy, its electrn still exists in the 1S rbital Instead, the aligned versin cmpared t the anti-aligned versin has hyperfine structure

5 Emissin - It is pssible fr hydrgen t jump frm its higher energy aligned state t the lwer energy antialigned state Very unlikely t happen spntaneusly: prbability f s 1 time it takes fr a single islated H atm t underg this transitin is ~ 10,000,000 yrs atmic cllisins decrease the time interval greatly allwing fr the spectral line t be bservable When it des happen, it releases a specific wavelength f light... Care t guess what that wavelength is?

6 The 21-cm Line: - The energy gap between the hyperfine structures directly crrespnds t the 21-cm wavelength ( MHz) This wavelength was predicted Hendrick C. van de Hulst in 1944 Discvered by Edward Mills Purcell and Harld Irving Ewen in Purcell_Hrn_Antenna.jpg/321px-Green_Banks_-_Ewen-Purcell_Hrn_Antenna.jpg

7 Detecting the 21-cm line: Full stry/histry can be fund here: fgdcs/hi21cm/ ephrn.html#thestry

8 The 21-cm Line: - S what's the pint? What can be dne with this infrmatin? First use f this was in 1952 where the first maps f neutral hydrgen in ur Galaxy were made These maps, using the dppler shift f the 1420 MHz spectral line, revealed the spiral structure f ur Galaxy ESO-VLT-Laser-pht-33a-07.jpg NGC_6384_HST.jpg

9 What Else? - Hydrgen is the mst abundant element in ur universe One f the first t frm during the aftermath f the big bang Therefre the spectral line f hydrgen can be used t identify cluds r surces utside f ur wn galaxy - By using the dppler shift, the relative mtins f bjects in ur universe can be tracked as well Nte: Neutral hydrgen is nt the same as mlecular hydrgen. Bth are fund in rughly the same amunt in ur galaxy, but mlecular hydrgen des nt have a clearly defined spectral line fr detectin.

10 Other Spectral Lines... - S if hydrgen can be identified by a specific frequency, can ther elements r cmpunds? Yes! Radi astrnmers have identified a lt ranging frm diatmic (2-atms) cmpunds all the way up t cmpunds made f mre than 10 atms Wikipedia lists nearly 200 f them!

11 Other spectral lines... Hydrxyl radical (OH) MHz Methyladyne (CH) MHz Frmaldehyde (H 2 CO) MHz Methanl (CH 2 OH) MHz Helium Istpe (3HeII) MHz Cyclprpenylidene (C 3 H 2 ) GHz Water Vapur (H 2 O) GHz Ammnia (NH 3 ) GHz On the left is a list f sme astrnmical chemicals and their crrespnding frequencies. Find radiatin at ne f these frequencies, and yu have fund a surce cntaining that mlecule.

12 Ok, we have fund smething... what else? - There is mre infrmatin embedded in the spectral line ther than the rigin's identity: Emitting/Absrbing Abundance/Density Directin and Velcity Rtatin Temperature and Pressure Electric / Magnetic Fields

13 Emissin / Absrptin - The presence f absrptin r emissin spectral lines depends n atms r mlecules making an energy transitin: Emissin - energy change t a lwer state Absrptin - energy change t a higher state

14 Emissin - Results in the actual emissin f light and a very specific and limited spectrum f light. (exneutral Hydrgen emits the wavelength f 21- cm)

15 Absrptin Happens when waves f an electrmagnetic wave (shwn in visible light) passes thrugh a cl gas. The gas rbs (absrbs) phtns at specific wavelengths t energize its electrns. Result Specific wavelengths drp ut f the therwise cntinuus spectrum.

16 Directin and velcity Velcity - By measuring the dppler shift f spectral lines in space yu can tell hw fast they are mving. - General Directin is measured based n which directin the shift is ccurring (red r blue) Lnger bservatins can yield a true path r rbit by measuring changes in the dppler shift ver time Pattern f mvement can als be determined in the same way (ex - milky way s spiral structure)

17 Temp. and Pressure - High temperatures and pressures can braden spectral lines. Typical Bradened - Due t high dppler shifts f individual mlecules

18 Temp. and Pressure cnt: - The mre unifrm in directin the atms are, the sharper the spike. Nt a cmmn ccurance - Matter tends t mve in all directins, the spectral spike is a measure f the average mtin f all atms in the area The higher the temperature, the faster mlecules mve, the brader the spike is ging t be

19 Electric / Magnetic fields - Depending n the differing directins and strengths f these fields, the bserved light waves will have undergne specific changes r have certain characteristics Ex - Stark Effect Plasma density (learn mre) Zeeman Effect Magnetic field (learn mre) Paschen-Back Effect Strng Magnetic Field ( learn mre

20 21-cm line reveals extra... - Mentined already: the hydrgen line revealed the spiral structure f ur galaxy via the dppler shift spectral line als allws fr a means t weigh areas f the csms - Interesting fact: the bservable mass frm stars and gas cannt accunt fr the Galaxy rtatin curves the Laws f Physics demand there must be mre mass invlved... sme f the best evidence fr the existence f dark matter!