A sample of 4208 objects with magnitude 15 < g ^ { * } < 22 and colors of main sequence A stars have been selected from 370 square degrees of Sloan Digital Sky Survey ( SDSS ) commissioning observations .
The data is from two long , narrow stripes , each with an opening angle of greater than 60 ^ { \circ } , at Galactic latitudes 36 ^ { \circ } < |b| < 63 ^ { \circ } on the celestial equator .
Relative photometric calibrations good to 2 % and consistent absolute photometry allows this uniform sample to be treated statistically over the large area .
An examination of the sample ’ s distribution shows that these stars trace considerable substructure in the halo .
Large overdensities of A-colored stars in the North at ( l,b,R ) = ( 350 , 50 , 46 \ > kpc ) and in the South at ( 157 , -58 , 33 \ > kpc ) and extending over tens of degrees are present in the halo of the Milky Way . Ivezic et al .
( 16 ) has detected the northern structure from a sample of RR Lyraes in the SDSS .

Using photometry to separate the stars by surface gravity , both structures are shown to contain a sequence of low surface gravity stars consistent with identification as a blue horizontal branch ( BHB ) .
Both structures also contain a population of high surface gravity stars two magnitudes fainter than the BHB stars , consistent with their identification as blue stragglers ( BSs ) .
The majority of the high surface gravity stars in the Galactic halo may be blue straggler stars like these .
A population of F stars associated with the A star excess in the southern structure is detected ( the F stars in the northern structure at 46 kpc would be too faint for the SDSS to detect ) .
From the numbers of detected BHB stars , lower limits to the implied mass of the structures are 6 \times 10 ^ { 6 } M _ { \odot } and 2 \times 10 ^ { 6 } M _ { \odot } , though one does not yet know the full spatial extent of the structures .
The fact that two such large clumps have been detected in a survey of only 1 % of the sky indicates that such structures are not uncommon in the halo .

Simple spheroidal parameters are fit to a complete sample of the remaining unclumped BHB stars and yield ( at r < 40 kpc ) a fit to a halo distribution with flattening ( c / a = 0.65 \pm 0.2 ) and a density falloff exponent of \alpha = -3.2 \pm 0.3 .