40 Eridani, Struve 518 (A, B, and C components)

Magnitude A / B / C
Separation A-BC/ AC
Position angle A-BC / AC
Spectral class A / B / C
Colour A / B / C
: Eridanus
: 04:15:49 / -07.37
: 4.4 / 9.5 / 11.2
: 83" / 80"
: 104°/ 117°
: K0.5V / DA 4 / M4.5
: Yellow / White / Red
Detail sketch:
Date / Time
Observing Location
Seeing / Transparency
Magnification / Field of View '
: 07/10/10 / 04:00
: Saint Amand de Coly
: 4 / 4
: Orion Optics UK 300mm
: 17mm Nagler Type 4
: 94x / 52'
40 Eridani

Observing Report

This beautiful triple is simply stunning. What a color contrast and difference in brightness. 40 Eridani is very easy to locate, because it's magnitude 4.4 A component is visible to the naked eye. From Rigel in Orion, first hop to Cursa Eridani. Then to the west to the couple Mu and Nu Eridani, and finally to 38 Eridani (Beid). To the southeast of 38 Eridani lies my target, 40 Eridani. All stars are naked eye stars, even from a suburban backyard.

40 Eridani is visible as a double, even at lowest magnification. The A component really looks yellow through the eyepiece. The B component looks white. The optimum view is with the 17mm Nagler. At this magnification the third component, a magnitude 11.2 faint and dark red star becomes visible at
7 o' clock, very close to the white star. They are separated only by a few arc-seconds. An amazing sight. There are no other significant field stars visible, so for my sketch I concentrated only on the triple star 40 Eridani.
Stacks Image 96

Image from Voyager by CapellaSoft


40 Eridanus is an interesting triple for more than one reason. The A, B and C components, all three dwarf stars, can be found in different regions of the Hertzschprung-Russell Diagram (HRD).

The magnitude 4.4 A component is a main sequence K0.5V yellow dwarf. The magnitude 9.5 B component is a DA4 hot white dwarf, found in the white dwarf region on the HRD.

The faint magnitude 11.2 M4.5eV red dwarf is on the main sequence also, but more to the lower right, where we find the cooler, fainter stars.
40 Eridani HR diagram
Difference in brightness between the three components

In the table to the right you find some basic data on the three components of 40 Eridani.

What you see in this table is the huge difference in brightness between the three dwarf stars. The brightest is the yellow dwarf, 40 Eridani A. But the B component, the white dwarf, is much hotter than the A component, so why doesn't the B component outshine the A component when observing it through my telescope? And why is the 3.300 degree C component so incredibly faint when observed visually?
40 Eridani
40 Eridani EMS
Well, to find an explanation for these questions, I first had a look at the electromagnetic spectrum. As you can see on the right, the visible light is only a very small part of the spectrum, and that's what I can see through my telescope.

But besides visible light, stars also emit forms of electromagnetic radiation that are invisible to our eyes, like infrared or ultraviolet. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves and infrared to the shortest wavelengths of ultraviolet, X-rays, and gamma rays.

The light of a star often comes very close to having a blackbody (or Planck) spectrum.
The white dwarf 40 Eridani B
When looking at the Blackbody spectra 1 and 2 I thought I understood why I see the yellow dwarf, with a temperature of 5.160 degrees, much better in the visual light, than the hot white dwarf with a temperature of 16.700 degrees.
The white dwarf peeks in the ultra violet (see blackbody spectrum 1), which means that a large part of the radiation is invisible to human eyes. The yellow dwarf peeks in the green part of the visible spectrum (see blackbody spectrum 2), so we see large parts of its radiation visually, with our own eyes.

But that's not the main reason. According to Professor James Kaler (which I contacted over the e-mail) even if the white dwarf 40 Eridani B would have its peak in the visible light, it would still appear fainter than the cooler 40 Eridani A. The main reason why this white dwarf is so faint is it incredible small size.
In the case of Eridani 40 B, the 0.44 solar mass is crammed into a sphere with a diameter of 1.48 earths! Very small indeed. Imagine an object of 1.5 earth diameters on a distance of 16 light-years.
40 Eridani Blackbody

Blackbody spectrum 1: a theoretical
source with a temperature of 16.700K (40 Eri B)

40 Eridani Blackbody

Blackbody spectrum 2: two theoretical sources
of 5.160K (yellow line, 40 Eri A) and 3.300K (red line, 40 Eri C)

On the image below you see a comparison I made between the Sun and the three components of 40 Eridani. The tiny white spec of Eridani 40 B shows how incredible small this hot and compact star is when compared to our sun, and the cooler A component. of 40 Eridani. The solar radius of the white dwarf is 0.02!
40 Eridani

The A,B and C component of 40 Eridani compared to the Sun.
Listed are the Solar Mass, Radius and Luminosity, as well as the temperature (T) in Kelvin

The red dwarf 40 Eridani C

The other question I wanted an answer for, is why the M4.5 red dwarf 40 Eridani C was visually even fainter than the very small white dwarf.

With the red dwarf it is a combination of reasons. It is small, but with a solar radius of 0.31 considerably larger than the white dwarf with a solar radius of 0.02. The red dwarf's luminosity however, is almost the same as the white dwarf's, even a bit more. But look at the temperatures. 40 Eridani C is with 3300 K much much cooler than 40 Eridanus B with 16700 K. On top of that, the M4 red dwarf has its peek radiation in the infrared, part of the spectrum which is invisible to our eyes (see Blackbody spectrum 2). Both the combination of lower temperature and peek radiation in the infrared, explain the very dim appearance of the red dwarf.

  • The the blackbody spectra were created with a free application which I downloaded from the website of the "PhET Interactive Simulations University of Colorado".
  • The image of the electromagnetic spectrum is by courtesy of: LASP/University of Colorado
  • Extreme Stars by James Kaler
  • Stars an their Spectra by James Kaler
  • The hundred greatest stars by James Kaler
  • An introduction to Sun and Stars by Green and Jones
  • The website "Stars" by James Kaler