Be­ware of im­i­ta­tions when hunt­ing the Pup Star, Sir­ius’s elu­sive com­pan­ion

Sky at Night Magazine - - FIELD OF VIEW -

The bright­est star in the night sky has a com­pan­ion, and try­ing to im­age or REVHUYH LW LV RQH RI WKH JUHDW FKDOOHQJHV for ama­teur astronomer­s.

The pri­mary is Sir­ius A, a white main se­quence star that ap­pears to shine at mag. -1.5. The al­pha star of Ca­nis Major the Great Dog, it’s also known as the Dog 6WDU ZKLFK LV ZK\ LWV KDUG WR QG com­pan­ion, the white dwarf Sir­ius B, has been dubbed the ‘Pup Star’.

2Q SDSHU 6LULXV % VKRXOG EH UHODWLYHO\ easy to see as it shines at mag. +8.5. In prac­tice, es­pe­cially from the UK, it’s any­thing but easy. This is down to the EULJKWQHVV DQG SUR[LPLW\ RI 6LULXV $ to­gether with the restricted al­ti­tude that WKH VWDU DFKLHYHV DERYH 8. KRUL]RQV )URP the UK, Sir­ius only man­ages to reach an al­ti­tude of 20° when due south. This means it tends to be affected by poor see­ing. You FDQ VHH HYLGHQFH RI WKLV RQ DQ\ FOHDU GDUN QLJKW 6LULXV DSSHDUV WR LFNHU DQG DVK GLI­IHUHQW FRORXUV 7KH LFNHULQJ LV GXH WR VHHLQJ ZKLOH WKH FRORXU DVKHV DULVH IURP D com­bi­na­tion of see­ing and at­mo­spheric dis­per­sion, which smear the star’s light into its com­po­nent spec­trum colours.

The good news is that Sir­ius B has been VORZO\ VHSDUDWLQJ IURP LWV SULPDU\ RYHU re­cent years. In 2019 it lies 11 arc­sec­onds from Sir­ius A. This means that now is an H[FHOOHQW WLPH WR DWWHPSW WR HLWKHU REVHUYH RU LPDJH LW

7KHUH KDYH EHHQ PDQ\ FODLPV WR KDYH im­aged Sir­ius B in the past few years. Quite D IHZ RI WKHVH KDYH EHHQ IDOVH FDSWXUHV ZKLFK KDYHQ W WDNHQ LQWR DFFRXQW WKH VHSDUDWLRQ GLVWDQFH LW LV RQ WKH VFDOH RI ap­par­ent plan­e­tary disc size. Jupiter’s DSSDUHQW VL]H LV FXUUHQWO\ RYHU WKUHH WLPHV the sep­a­ra­tion dis­tance! Many of the LPDJHV DFWXDOO\ UHFRUG DQRWKHU IDLQW HOG star that ap­pears close to Sir­ius but much fur­ther out than Sir­ius B. One way to UHPRYH DQ\ GRXEW LV WR LPDJH DQ REMHFW RI com­pa­ra­ble size to 11 arc­sec­onds to cal­i­brate your imag­ing setup. This could be D SODQHW D OXQDU IHD­WXUH RU D PRUH HYHQO\ PDWFKHG GRXEOH VWDU )RU H[DPSOH WKH VWDU Nair al Saif (Iota Ori­o­nis) at the bot­tom of Orion’s Sword con­sists of a mag. +2.8/+7.7 pair sep­a­rated by 11 arc­sec­onds.

9DULRXV PHWKRGV KDYH EHHQ SXW IRUZDUG to as­sist with the sight­ing of Sir­ius B. One is to place Sir­ius off the south­west edge of \RXU HOG RI YLHZ VR LWV EULOOLDQFH LV KLGGHQ $QRWKHU LV WR REVHUYH RU LPDJH 6LULXV ZKHQ the sky is still bright at twilight. The brighter back­ground re­duces con­trast and should, in the­ory, make Sir­ius B that much HDVLHU WR VSRW +RZHYHU \RX WU\ LW JRRG luck and let us know how you get on.


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