1. Toomre Sequence
In the 1970s, Alar and Juri Toomre illustrated the merger process of spiral galaxies using images of nearby galaxies. This has become known as the 'Toomre Sequence'.

2. Toomre Sequence 1 (NGC 5426 & 5427) 15’ x 15’
In the first image of the Toomre sequence shown above, two galaxies (NGC5426/5427) approach each other for the first time.

3. Toomre Sequence 2 (NGC 4038 & 4039) 15’ x 15’
The following image (NGC4038/4039) shows the beginnings of the actual merger process, with the formation of long tidal tails.

5. Toomre Sequence 3 (NGC 7252) 15’ x 15’
The central merging is basically complete, but evidence for tidal tails and shells persist.

7. Toomre Sequence 4 (NGC 3610) 15’ x 15’
Tidal Tails and shells slowly dissipate, the galaxy starts to resemble a normal elliptical galaxy though with some evidence of disturbance.

8. Toomre Sequence 5 (M 89) 15’ x 15’
Eventually all that remains is a merger remnant which shows no outward sign of its violent origins. Individual galaxies are expected to pass through this sequence in approximately 500 million years.

9. Toomre Sequence 6 (M 89) 15’ x 15’
After this period of time, the outward signs of the merger will have faded, but techniques such as unsharp masking (made famous by David Malin) can still detect evidence for mergers.
This technique has brought a disrupted dwarf galaxy (middle right) and two shells (upper right and lower left) into sharp contrast, features which indicate that this galaxy has undergone a recent merger.

10. Toomre Sequence 1 (NGC 5426 & 5427) 15’ x 15’
In the first image of the Toomre sequence shown above, two galaxies (NGC5426/5427) approach each other for the first time.

11. Toomre Sequence 2 (NGC 4038 & 4039) 15’ x 15’
The following image (NGC4038/4039) shows the beginnings of the actual merger process, with the formation of long tidal tails.

12. Toomre Sequence 3 (NGC 7252) 15’ x 15’
The central merging is basically complete, but evidence for tidal tails and shells persist.

13. Toomre Sequence 4 (NGC 3610) 15’ x 15’
Tidal Tails and shells slowly dissipate, the galaxy starts to resemble a normal elliptical galaxy though with some evidence of disturbance.

14. Toomre Sequence 5 (M 89) 15’ x 15’
Eventually all that remains is a merger remnant which shows no outward sign of its violent origins. Individual galaxies are expected to pass through this sequence in approximately 500 million years.