puzzled

[darnun]

Question? How do astronomers know a star or nebula, etc is
say 4 or 400 light years away.
Also how do they know an asteroid is between 4 or 4oo metres in size and
should miss earth by say 5 lunar distances?

[Blue Skies (Jacquie)]

This is pretty big topic.

Measuring star distances can be done in a variety of ways. The closer ones are done by the parallax method, the old basic geometry. However there are many others (I have a list next to me that was done by a professional astronomer and even he puts "various other techniques" at the bottom. They can find variable stars, such as Cepheids, RR Lyrae stars and eclipsing binaries.

For globular clusters they plot all the stars out on a Hertzpsrung-Russell diagram and that gives them the clue (somehow, not sure about this one but I know it gives age of the cluster as well!) and if they know what type of star it is and it's mass they can use the inverse square law for light diffusion (ok someone else has to help me here!!)

Galaxies are done by redshift of their spectrum.

For solar system objects you need to get a minimum of 3 positions and then some maths is done to compute the orbit. I'm still to learn how that all works as well, but the more observations the better the result.

Size of asteroids is fuzzy, as they can only go on how much light it reflects - ideally a large asteroid would reflect lots of light, but what if its got a dark surface? If the asteroid has a moon then you're laughing because the interaction of the two bodies gives the mass away.

Hopefully this will get the ball rolling and get some clearer answers coming in.

[bojan]

Originally Posted by Blue Skies

For solar system objects you need to get a minimum of 3 positions and then some maths is done to compute the orbit. I'm still to learn how that all works as well, but the more observations the better the result.

Hi Jacquie,
If you are interested in this topic, have a look at this program below (written in BASIC) that computes preliminary orbit of Solar system object.
The program is written by Erich Karkoschka (from Arizona University) back in '80, during a Summer Astronomical camp in Adriatic... I believe it is in public domain since it was published in a summary journal after the occasion.
I tested it (by taking positions of planet from ephemerides, then calculated orbit, then calculated positions from that orbit) and it works pretty accurately, however it must be compiled and run in HU-BASIC (that works with 16-digit mantissa) or better otherwise the rounding errors accumulate and the results are not valid, or iteration does not converge at all.

Positional data in a form of Date (string), Time [hours] RA, DEC are at the end of the program.
When finished, the program prompts for saving results in a file.
I found this way convenient when testing....


5 DEFDBL A-H
7 DEFDBL O-Z
10 CLS : PRINT " Program for estimating preliminary orbital "
11 PRINT "elements for members of Solar system from 3 observations": PRINT
12 PRINT "Method developed by Erich Karkoschka, 1987?)": PRINT
14 DIM M$(12): DIM A$(3)
15 M$(1) = "Jan": M$(2) = "Feb": M$(3) = "Mar": M$(4) = "Apr"
16 M$(5) = "May": M$(6) = "Jun": M$(7) = "Jul": M$(8) = "Aug"
17 M$(9) = "Sep": M$(10) = "Oct": M$(11) = "Nov": M$(12) = "Dec"
20 LN = .434294481#: ST = 57.29577951#: DN = .99726957#
22 PI = 3.141592654#: P2 = 2 * PI: EK = 23.44579 / ST
24 SE = SIN(EK): CE = COS(EK)
30 PRINT : REM : INPUT "Name of object ? "; N$
40 DEF FNASN (X#) = ATN(X# / SQR(-X# * X# + 1))
42 DEF FNACS (X#) = -ATN(X# / SQR(-X# * X# + 1)) + PI / 2

45 REM GOTO 130

50 FOR K = 1 TO 3: REM *** TEST***

60 REM ************************
70 READ A$(K), T, RH(K), RM(K), DD(K), DM(K): GOSUB 1940: NEXT
80 REM ************************

85 GOTO 200

90 REM *** INPUT DATA ***
130 HOME: FOR K = 1 TO 3
140 PRINT K; ".": PRINT : INPUT "Datum: (G MJ D) "; A$(K): PRINT
150 IF MID$(A$(K), 5, 1) <> " " THEN 140
160 INPUT "Vrijeme: (UT: Hr,min): "; TH, TM: PRINT : T = TH + TM / 60
170 INPUT "Ra: (h,min,sec )"; RH(K), RM(K), RS(K): PRINT
180 INPUT "Dec: (deg,min,sec )"; DD(K), DM(K), DS(K): PRINT
190 GOSUB 1940: NEXT K
200 CLS : PRINT N$: PRINT : FOR K = 1 TO 3: PRINT : PRINT " "; K; ". ";
210 PRINT A$(K); " (JD="; T(K); ")": PRINT
220 PRINT "Ra= "; RH(K); "hr "; RM(K); "min "; " Dec= "; DD(K); "deg "; DM(K); "min": PRINT
230 NEXT: PRINT "Are the input data O.K. ? (Y/N ?)"
235 INPUT Y$: IF Y$ <> "Y" AND Y$ <> "y" THEN 30

240 O$ = "Eliptic iteration"

250 PRINT : PRINT " Eqx 1950/2000 ?(1/2)": INPUT S$: IF S$ = "1" OR S$ = "2" THEN 270
260 GOTO 250
270 IF S$ = "1" THEN EQ = 1950: GOTO 290
280 IF S$ = "2" THEN EQ = 2000: GOTO 290
290 CH = 0: ON ERROR GOTO 2030

300 REM * Calculation *

310 REM * 1 *
320 FOR K = 1 TO 3
330 B(K) = FNASN(CE * SIN(D(K)) - SE * COS(D(K)) * SIN(R(K)))
340 L(K) = 2 * ATN((SIN(D(K)) * SE + COS(D(K)) * CE * SIN(R(K))) / (COS(B(K)) + COS(D(K)) * COS(R(K))))
350 IF L(K) < 0 THEN L(K) = L(K) + P2
360 IF EQ = 2000 THEN L(K) = L(K) + .7 / ST

370 NEXT

380 REM * 2 *
390 M0 = (T(2) - 2445338.087#) * .985626283# / ST
400 V0 = M0 + (1.915# * SIN(M0) + .02 * SIN(2 * M0)) / ST
410 L0 = V0 - 77.126 / ST
420 R0 = .9997200000000001# / (1 + .016716# * COS(V0))
430 LV = (M0 + V0) / 2 + 12.874 / ST
440 V0 = .01720209895# / R0
450 IF EQ = 1950 THEN L0 = L0 - .7 / ST: LV = LV - .7 / ST

460 REM * 3 *
470 T1 = T(1) - T(2)
480 T3 = T(3) - T(2)

490 REM * 4 *
500 L(1) = L(1) - L(2)
510 L(3) = L(3) - L(2)
520 L0 = L0 - L(2)
530 LV = LV - L(2)

540 REM * 5 *
550 FOR K = 1 TO 3 STEP 2
560 S(K) = 1 / (SIN(B(2)) * TAN(B(K)) + COS(B(2)) * COS(L(K)))
570 X(K) = -S(K) * SIN(L(K))
580 Y(K) = S(K) * (COS(B(2)) * TAN(B(K)) - SIN(B(2)) * COS(L(K)))
590 NEXT

600 REM * 6 *
610 V = SIN(B(2)) / TAN(L0)
620 W = 2 * COS(B(2)) * COS(L0)

630 REM * 7 *
640 F = -(Y(3) - V * X(3)) / (Y(1) - V * X(1))

650 REM * 8 *
660 G = (F * X(1) + X(3)) / (F * T1 * T1 + T3 * T3)

670 REM * 9 *
680 H = (1 / T3 + 1 / ABS(T1)) / (1 + F)

690 REM * 10 *
700 XP = (X(3) - G * T3 * T3) * H
710 YP = (Y(3) - G * V * T3 * T3) * H
720 ZP = (1 / ABS(T1) - F / T3) / (1 + F)

730 REM * 11 *
740 MU = .0002958#
750 H1 = COS(B(2)): H2 = R0 * COS(L0)
760 H3 = R0 * SIN(L0)
770 H4 = SIN(B(2))
780 H5 = ZP * COS(B(2)) - YP * SIN(B(2))
790 H6 = V0 * COS(LV)
800 H7 = V0 * SIN(LV)
810 H8 = ZP * SIN(B(2)) + YP * COS(B(2))
820 IF LEFT$(O$, 1) = "P" THEN GOTO 920

830 REM * Eliptic iteration *
840 G = (2 * G * R0 * R0 * R0) / (MU * SIN(L0))
850 D = 1.5: CLS : PRINT TAB(10); O$


860 FD = ((1 + D * D - W * D) ^ (-1.5) - 1) / D + G
870 FS = -3 * (2 * D - W) * (1 + D * D - W * D) ^ (-2.5) / 2 / D - (1 + D * D - W * D) ^ (-1.5) / D / D + (1 / D / D)
880 D1 = D - FD / FS
890 IF ABS(D - D1) > .0000000000001# THEN D = D1: GOTO 860
900 D = R0 * D
910 GOTO 1030

920 REM * Parabolic iteration *
930 D = 1.2: PRINT TAB(10); : PRINT O$: PRINT TAB(10);
940 U = SQR((D * H1 - H2) * (D * H1 - H2) + H3 * H3 + D * D * H4 * H4)
950 US = (D * (H1 * H1 + H4 * H4) - H1 * H2) / SQR((D * H1 - H2) * (D * H1 - H2) + H3 * H3 + D * D * H4 * H4)
960 FS = US * (D * H5 - H6) * (D * H5 - H6) + U * 2 * (D * H5 * H5 - H5 * H6)
970 FS = FS + US * (-D * XP - H7) * (-D * XP - H7) + U * 2 * (D * XP * XP + XP * H7)
980 FS = FS + US * D * D * H8 * H8 + U * 2 * D * H8 * H8
990 FD = U * (D * H5 - H6) * (D * H5 - H6) + U * (-D * XP - H7) * (-D * XP - H7) + U * D * D * H8 * H8 - 2 * MU
1000 D1 = D - FD / FS
1010 IF ABS(D1 - D) > .0000000000001# THEN D = D1: GOTO 940
1020 REM

1030 REM * 12 *
1040 ON ERROR GOTO 2030
1050 X = D * H1 - H2
1060 Y = -H3
1070 Z = D * H4
1080 R = SQR(X * X + Y * Y + Z * Z)

1090 REM * 13 *
1100 PX = D * H5 - H6
1110 PY = -D * XP - H7
1120 PZ = D * H8
1130 V = SQR(PX * PX + PY * PY + PZ * PZ)
1140 Y1 = R * V * V - 2 * MU

1150 REM * 14 *
1160 XS = Y * PZ - Z * PY
1170 YS = Z * PX - X * PZ
1180 ZS = X * PY - Y * PX
1190 H = SQR(XS * XS + YS * YS + ZS * ZS)
1200 IF LEFT$(O$, 1) = "P" THEN AO = 0: EO = 1: NO = 0: QO = H * H / 2 / MU: GOTO 1290

1210 REM * 15 *
1220 AO = 1 / (2 / R - V * V / MU)

1230 REM * 16 *
1240 NO = SQR(MU / AO / AO / AO)

1250 REM * 17 *
1260 EO = SQR(1 - H * H / MU / AO)
1270 QO = AO * (1 - EO)

1280 REM * 18 *
1290 IO = FNACS(ZS / H)

1300 REM * 19 *
1310 OO = L(2) + SGN(XS) * FNACS(-YS / H / SIN(IO))
1320 IF OO < 0 THEN OO = (OO + P2)
1330 IF OO > P2 THEN OO = (OO - P2)

1340 REM * 20 *
1350 VO = SGN(X * PX + Y * PY + Z * PZ) * FNACS((H * H / MU / R - 1) / EO)

1360 REM * 21 *
1370 WO = SGN(Z) * FNACS((Y * XS - X * YS) / R / H / SIN(IO)) - VO
1380 IF WO < 0 THEN WO = WO + P2
1390 IF WO > P2 THEN WO = WO - P2
1400 IF LEFT$(O$, 1) = "P" THEN T = T(2) - (R * SIN(VO) / 3 / H) * (4 * QO - R * COS(VO)): GOTO 1470

1410 REM * 22 *
1420 EC = 2 * ATN(SQR((1 - EO) / (1 + EO)) * TAN(VO / 2))

1430 REM * 23 *
1440 MO = EC - EO * SIN(EC)

1450 REM * 24 *
1460 T = T(2) - MO / NO
1470 JD = T: GOSUB 1800

1480 REM * Printing resuts *
1490 REM * K *
1500 PRINT TAB(10); : PRINT N$: PRINT "Orbital elements :"
1505 PRINT : PRINT
1510 IF AO <> 0 THEN PRINT " a ="; AO; " AU"
1520 PRINT " q ="; QO; " AU"
1530 PRINT " e ="; EO
1540 IO = IO * ST: PRINT " i ="; IO; " deg"
1550 OO = OO * ST: PRINT " o ="; OO; " deg"
1560 WO = WO * ST: PRINT " w ="; WO; " deg"
1570 PRINT " T ="; JJ; " "; M$(MM); " "; TT; " (JD ="; JD; ")": PRINT
1580 JD = JD - 2433282

1590 PRINT "Do you want to save those data on disk ? (Y/N ?)"
1595 INPUT Y$: IF Y$ <> "Y" THEN 1640
1597 PRINT
1600 'D$ = CHR$(4): PRINT D$; "OPEN"; N$
1610 'PRINT D$; "WRITE"; N$
1620 'PRINT QO: PRINT EO: PRINT IO: PRINT OO: PRINT WO: PRINT JD
1630 'PRINT D$; "CLOSE"; N$

1640 IF LEFT$(O$, 1) = "P" THEN 1670
1650 PRINT
1660 PRINT " n ="; NO * ST; " (deg/24h)"
1670 PRINT : PRINT " Anomalies for T2 ( "; A$(2); " )": PRINT
1680 PRINT " True anomaly An ="; VO * ST; " deg": PRINT
1690 IF LEFT$(O$, 1) = "P" THEN 1710
1700 PRINT " Excentr. anomaly E ="; EC * ST; " deg": PRINT
1710 PRINT " Mean anomaly M ="; MO * ST; " deg"
1712 PRINT : PRINT "More (Y/N) ?"
1717 INPUT Y$: IF Y$ = "Y" THEN RUN
1720 STOP

1730 REM * date -> jul. day *
1740 REM * *
1750 IF MM > 2 THEN C = JJ: D = MM
1760 IF MM <= 2 THEN C = JJ - 1: D = MM + 12
1770 A = INT(C / 100): B = 2 - A + INT(A / 4)
1780 JD = INT(365.25# * C) + INT(30.6001# * (D + 1)) + TT + 1720994.5# + B
1790 RETURN

1800 REM * Jul. day -> date *
1810 JD = JD + .5: Z = INT(JD): F = JD - INT(JD)
1820 IF Z < 2299161 THEN A = Z
1830 IF Z > 2299161 THEN A = INT((Z - 1867216.25#) / 36524.25): A = Z + 1 + A - INT(A / 4)
1840 B = A + 1524
1850 C = INT((B - 122.1) / 365.25)
1860 D = INT(365.25 * C)
1870 E = INT((B - D) / 30.6001)
1880 TT = B - D - INT(30.6001 * E) + F
1890 IF E < 13.5 THEN MM = E - 1
1900 IF E > 13.5 THEN MM = E - 13
1910 IF MM > 2.5 THEN JJ = C - 4716
1920 IF MM < 2.5 THEN JJ = C - 4715
1930 RETURN

1940 REM

1950 REM * Date from A$ into JJ,MM,TT *
1960 JJ = VAL(LEFT$(A$(K), 4)): MM = VAL(MID$(A$(K), 6, 2)): TT = ABS(VAL(MID$(A$(K), 8, 26)))
1970 TT = TT + T / 24
1980 GOSUB 1730
1990 T(K) = JD
2000 R(K) = 15 * (RH(K) + RM(K) / 60 + RS(K) / 3600) / ST
2010 SG = SGN(DD(K)): D(K) = (DD(K) + SG * DM(K) / 60 + SG * DS(K) / 3600) / ST
2020 RETURN

2030 REM * Changes *
2040 IF CH = 1 THEN GOTO 2090
2050 CH = 1: ON ERROR GOTO 10
2060 PRINT " ORBIT IS NOT ELIPTIC"
2070 O$ = "Parabolic iteration"
2080 GOTO 300

2090 REM * NOT POSSIBLE ! ***
2100 REM
2110 FLASH: PRINT " ORBIT DOES NOT EXIST !"
2120 STOP

2130 REM
2140 REM ***********************
2150 REM T E S T
2160 REM ***********************
2170 DATA "1983-05-29",10.08,17,57.8,-3,26
2180 DATA "1983-06-05",0.0,17,52.5,-3,21
2190 DATA "1983-06-15",0.0,17,43.1,-3,29

[AstralTraveller (David)]

Originally Posted by Blue Skies

Size of asteroids is fuzzy, as they can only go on how much light it reflects - ideally a large asteroid would reflect lots of light, but what if its got a dark surface? If the asteroid has a moon then you're laughing because the interaction of the two bodies gives the mass away.

There is a way to get a direct measurement of an asteroids size - and amateurs are leading the way. It's easy to know where an asteroid is (how far away) and how fast it is moving. So if one times how long it takes to pass in front of a star one can work out the length of the chord across the asteroid. If more than one chord is observed by observers at appropriate spacing then a good estimate of the size and shape of the asteroid is obtained.

In practice the asteroid is normally much fainter than the star, which is bright enough to be accessible in amateurs' telescopes, and so the observer sees the star blink off for a few seconds. Observers are typically using about 20cm scopes equiped with a video camera and a bit of other gear to give a time signal.

There is a small but active group of observers in Aust and NZ who are getting positive results fairly regularly. The following links may be of interest.

http://occsec.wellington.net.nz/
http://www.hristopavlov.net/OccultWa...ltWatcher.html
http://www.lunar-occultations.com/iota/occult4.htm

[DJDD]

the podcasts, Astronomy 161 and Astronomy 162, go into this in depth.


found on the Resources > podcasts page: http://www.iceinspace.com.au/podcasts.html

good to listen to when on the way to/from work.

cheers

[Blue Skies (Jacquie)]

Hmm, seems my response got more attention than the original question! Isn't anyone else going to help darnun out?

Originally Posted by AstralTraveller

There is a way to get a direct measurement of an asteroids size - and amateurs are leading the way.

Gosh, yes, how could I forget! You don't need to convert me to the occultation cause, I'm already on the bandwagon (have been for years) and very slowly working my way towards joining in.

I also thought about photometric modelling while I was driving around at work today - where light curves of the asteroid are collected and put into a model. Plus there is radar for close passing asteroids, such as Toutatis.

[Blue Skies (Jacquie)]

Originally Posted by bojan

Hi Jacquie,
If you are interested in this topic, have a look at this program below (written in BASIC) that computes preliminary orbit of Solar system object.
The program is written by Erich Karkoschka (from Arizona University) back in '80, during a Summer Astronomical camp in Adriatic... I believe it is in public domain since it was published in a summary journal after the occasion.

Thanks, bojan. I'm not sure if it will help, though. What I was thinking about was when people, like darnun, ask "how?", something I get asked from time to time as well as a local 'expert'. As I've already indicated, I know at least three observations/positions are needed, the more the merrier. Most people, including myself, are not so mathematically minded and are looking for a non-numerical description of the process.

[darnun]

Thankyou Jacquie (blue skyes)

Your post has shed some light on one of my questions, namely the
part on asteroid sizes.
As for anything else, well........I think I will stick to just peering thru
my telescope and leave the hard answers to the experts.

[glenc (Glen)]

The accurate distances to 120,000 stars were measured by Hipparcos.
http://en.wikipedia.org/wiki/Hipparcos

"The final Hipparcos Catalogue (120,000 stars with 1 milliarcsec level astrometry) and the final Tycho Catalogue (more than one million stars with 20-30 milliarcsec astrometry and two-colour photometry) were completed in August 1996. The catalogues were published by ESA in June 1997."

In 2011 Gaia will do even better.
"Gaia will compile a catalogue of approximately one billion stars to magnitude 20. Its objectives comprise:

• astrometric (or positional) measurements, determining the positions, distances, and annual proper motions of stars with an accuracy of about 20 µas (microarcsecond) at 15 mag, and 200 µas at 20 mag
• spectrophotometric measurements, providing multi-epoch observations of each detected object
• radial velocity measurements."

http://en.wikipedia.org/wiki/Gaia_mission

[bojan]

Apparently, Hipparcos did not very well with douple stars (spectroscopic and close binaries).
This is became pretty obvious for me when I was playing with Celestia.. I wanted to see what Pleiades look like when viewed from side.. so I went to the same distance from Sol and looked at them and the cluster was grossly elongated along the line of sight (towards Sol), something I did not expect but I immediately concluded something went very wrong with measurement accuracy..
Hope Gaia will do better...

[Enchilada]

Originally Posted by bojan

Apparently, Hipparcos did not very well with double stars (spectroscopic and close binaries).
This is became pretty obvious for me when I was playing with Celestia.. I wanted to see what Pleiades look like when viewed from side.. so I went to the same distance from Sol and looked at them and the cluster was grossly elongated along the line of sight (towards Sol), something I did not expect but I immediately concluded something went very wrong with measurement accuracy..
Hope Gaia will do better...

Actually this is only partly true. Hipparcos design involved measures by interferometry. The problem is separations was worst at around 0.7 arcsec, and poor up to 2 arcsec. Anything above 2.5 arcsec has no difficulties in accuracies averaged at about 0.003 arcsec. Several doubles below 0.3 arcsec have similar accuracies - with error between 0.5% to 1% - better than visual observations. These errors are also complicated by difference in magnitude (Delta-m) between the components. Also the proximity of the star made defining the photometric centres problematic, hence the errors in the parallaxes.
The reasons for the cluster is because of significant systematic errors in the observations. In case of the Pleiades , if you look at the error bars. We know the RA and Dec very well, parallaxes are much worst. Still better accuracy than what we had before it by a factor of ten or more at least.

[ngcles]

Hi Darnun & All,

I think the main reason why the replies have been a bit scattered is because the whole distance scale thing is very complex and it is a "cantilevered" system to an extent that also has some cross-checks available. It takes a fair bit of typing to get across. Here goes:

I won’t touch on solar-system objects – that has been answered by Dave & others but outside that I’ll attempt a summary that’ll hopefully give you a basis to explore the subject elsewhere for really complete answers.

We find the distance to the nearest stars using parallax. In the early days observers would make very careful observations of stars presumed to be close-by with a micrometer and then over the course of a year (the period of Earth’s orbit) make further observations to see if the star shifts relative to the background stars. If you can measure the shift and therefore the angle of the parallax shift, we know the diameter of the Earth’s orbit as a baseline then we can find the distance to the star using trigonometry. This method’s accuracy used to be limited to about 30-40ly because of the accuracy of the measuring equipment, the atmosphere and error bars.

The Hipparcos mission because it was in orbit above the atmosphere and because of the improvement in technology could make measures using the same basic method much, much more accurately out to far greater distances. The distance to tens of thousands of stars to about 500 ly is known with good accuracy – further out the error bar increases. Gaia will improve upon this by a couple of orders of magnitude in precision and numbers of stars when it flies in a few years.

Now, knowing the distance to so many stars has allowed us to strongly confirm our stellar evolution theories. So, even if we see stars far outside the ability to measure their distances directly, if we take their spectrum, temperature, brightness and determine their luminosity class. From that we can then from stellar theory, work out how far they must be away from their intrinsic luminosity as indicated by stellar theory. Not completely accurate but works well. We can look at a cluster, take spectra of many stars therein get several estimates on distances from the many stars (hopefully they will all agree within reason), average them and get a pretty good overall estimate to a cluster.

Most importantly, the Cepheid variable stars are a wonderful standard candle – their intrinsic luminosity is directly and simply related to the period they brighten and fade. We have very precisely measured the distance to many Cepheids using parallax, so we know exactly how intrinsically luminous the Cepheid’s are. A very, very useful property. Because they are bright stars, we can see them directly in other galaxies out to about 70 million ly using HST and other very large professional ‘scopes. All we have to do is observe and identify them in the other galaxy and then measure their period exactly. This tells us precisely how luminous that star is. Once we know that, we compare it to how luminous it actually is in the observation and this provides the distance to that star (and therefore the galaxy) pretty accurately.

We use a similar method with globular clusters but with a different variable star – the RR Lyrae variables (a type of mini-Cepheid).

Now we move to red-shift – a galaxy’s recessional velocity more or less equates to its distance away from us in an expanding universe. If we can use Cepheids to accurately determine the distance to many galaxies within 100 million ly, we can then accurately calibrate the red-shift scale. This produces the so called Hubble constant – the main reason we have HST. These observations tell us that the Hubble constant is +71kms/sec/mpc (+/- 2). Knowing this we can look at distant galaxies, take their spectra, measure the redshift in kms/sec and then applying the Hubble Constant, estimate the distance pretty well.

There is also one type of supernova that is particularly useful in all this – the Type Ia supernovae (ie a detonating white dwarf star) All the Type Ia supernovae essentially have exactly the same intrinsic brightness (for complex reasons). They are superb standard candles out to about 1 billion ly and we see them in other galaxies all the time. Because they are all uniformly bright, we can use them like a Cepheid as a measure in itself and to suppor/confirmt a red-shift estimate, and also calibrate/confirm a Cepheid derived distance.

Beyond this, it gets a bit more rubbery and to an extent depends upon exactly how quickly the acceleration of the Universe’s expansion is measured to be and whether it is uniform in both space and time. Another indicator at very large scales is the largest galaxy in the cluster rule. It is a fair assumption that in massive and populous galaxy clusters, the very largest galaxy will be comparable in brightness and mass to other “brightest galaxies” in nearby clusters that we can more directly measure the distance to. We are in effect using the most massive and brightest galaxy in a cluster as another standard candle in itself.

That’s it, (as I understand it – and I’m no astrophysist) in a nutshell! There are other indirect methods too which are all used as confirming (hopefully) evidence of the more direct methods.

have I tried not to make it too long or complex --hope it helps in some way! It is a big subject.



Best wishes to you and everyone else for the season,


Les D