THE SKY GUIDE CHALLENGE
Beware of imitations when hunting the Pup Star, Sirius’s elusive companion
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.